UNIVERSITY  OF  CALIFORNIA 

ANDREW 

SMITH 

HALLIDIL: 


ELECTRICITY   AND    MAGNETISM 

AND   THEIR   APPLICATIONS 


/s 

AN 

I        ELEMENTARY   BOOK 

ON 

felectricity  and  Magnetism 

AND   THEIR   APPLICATIONS 


A  TEXT-BOOK  FOR  MANUAL  TRAINING  SCHOOLS  AND   HIGH 
SCHOOLS,  AND  A  MANUAL  FOR  ARTISANS,  APPREN- 
TICES,  AM;    liOME    READERS 


DUGALD   C.   JACKSON,   C.E. 

Professor  of  Elect t  ical  Engineering,   I  'n/rer \//v  </  Wi 
Member  of  the  American   Institute  of  Electrical  Engineers,  etc. 
AND 

JOHN    PRICE   JACKSON,    M.E. 

Professor  of  Electrical  Engineer  ing,   reunsvlranici  Hate   College 
Member  of  the  American  Institute  of  Electrical  Engineers,  etc. 


THE    MACMILLAN    COMPANY 

LONDON  :  MACMILLAN   &  CO.,   LTD. 
1902 

All  rights  reserved 


' 
3 


'•  ^   '»  COPYRIGHT'  19(V?  " 

BY  THE  MACMI'LL!^  COMPANY. 


Set  up  and  electrotype^  Jaiiuavy,"  -^0 

HALLIDIE 


Kovtoooti 

J.  S.  Gushing  &  Co.  -  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


PREFACE 

WHILE  this  book  is  more  especially  intended  for  an  elementary  text- 
book, it  is  believed  that  it  will  be  interesting  to  all  readers  who  have  a 
taste  for  science  and  that  it  will  prove  a  useful  manual  for  apprentices 
and  artisans.  Every  effort  has  been  exerted  to  make  it  clear,  forceful, 
and  of  strict  scientific  accuracy,  though  it  is  written  in  reasonably  collo- 
quial language. 

The  book  is  essentially  the  outcome  of  the  authors'  belief  that  ele- 
mentary physical  science  may  properly  be  —  nay,  should  only  be  — 
taught  in  an  atmosphere  filled  with  the  inspiration  gathered  from  an 
interest  in  every-day  occurrences.  (Physics  is  a  science  of  our  daily 
life  and  experiences.)  If  this  is  a  very  ordinary  belief,  it  surely  is  one 
much  honored  in  the  breach. 

The  masters  of  the  theory  and  of  the  practice  of  teaching,  from  the 
sixteenth  century  to  the  present,  have  held  that  all  intellectual  acquire- 
ment must  come  to  us  through  the  senses,  and  that  we  can  reason  upon 
the  abstract  only  by  reaching  out  from  the  concrete  which  is  already 
grasped.  Moreover,  elementary  instruction  should  be  made  interesting 
to  the  pupils,  and  the  matter  should  be  presented  in  an  order  and  in  a 
manner  which  will  result  in  the  readiest  and  iriost  complete  assimilation. 

These  principles  of  good  teaching  point  to  an  overthrow  of  the  tradi- 
tional academic  order  and  abstract  presentation  in  elementary  instruc- 
tion in  physical  science  and  the  substitution  therefor  of  a  rational 
presentation  of  applied  science.  The  rational  order  of  instruction  here 
is  one  which  follows  very  nearly  in  the  same  sequence  as  the  order 
of  discoveries  and  industrial  applications.  Industrial  development  is 
usually  along  the  line  of  march  from  the  simple  to  the  complex,  and 
this  path  must,  in  the  main,  be  followed  to  stimulate  the  most  effective 
assimilation  by  the  pupil. 

V 

96064 


yi  PREFACE 

The  writers  have  herein  attempted  to  treat  a  division  of  physical 
science  in  the  manner  suggested.  The  order  of  the  book  is  from  the 
simple  to  the  complex,  and  the  pages  keep  in  reasonably  close  touch 
with  the  more  or  less  common  experiences  of  the  pupils  for  whom  the 
book  is  intended,  or  with  the  knowledge  which  comes  by  reasoning 
directly  from  those  experiences. 

The  treatment  has  been  planned  with  the  purpose  of  introducing,  at 
desirable  places  in  the  regular  text,  elementary  explanations  of  certain 
scientific  principles  which  are  not  a  part  of  the  immediate  subject  of  the 
book;  which  cannot  be  assumed  to  be  already  a  part  of  the  pupil's 
knowledge ;  but  which  must  be  understood  by  the  pupil  before  he  can 
properly  grasp  the  subject-matter  in  hand.  The  plan  pursued  is  unusual, 
as  it  places  the  statement  of  these  principles  at  the  points  of  their  appli- 
cation instead  of  in  the  introductory  part  of  the  book  ;  but  it  is  believed 
to  be  a  proper  plan  for  an  elementary  book,  since  it  encourages  the 
pupil  to  base  all  of  his  reasoning  directly  upon  foundation  knowledge. 

Extended  use  of  analogies  has  been  made  throughout  the  book,  but 
especially  in  its  earlier  part.  Definitions  of  important  units  are  often 
left,  in  the  earlier  part  of  the  book,  to  be  inferred  from  them  ;  but  in  all 
such  cases  the  exact  definitions  are  found  on  later  pages.  This  accords 
with  the  usual  order  of  acquiring  a  knowledge  of  physical  units  by  chil- 
dren, and  should  add  vigor  and  zest  to  the  pupil's  pursuit  of  intellectual 
acquirement.  It  is  the  constant  desire  of  the  writers  to  interest  the 
pupil,  and  to  stimulate  him  to  an  active  inquiry  into  the  principles  and 
laws  which  underlie  physical  phenomena,  and  to  bring  him  to  a  reason- 
ably vivid  physical  conception  of  the  characteristics  of  the  phenomena. 

It  is  desirable  that  suitable  laboratory  practice  shall  accompany  the 
use  of  the  book  in  the  schools.  This  may  consist  of  experimental 
examinations  of  electric  and  magnetic  phenomena  and  the  performance 
of  the  simpler  electrical  measurements.  The  laboratory  instruction 
should  be  designed  to  aid  the  pupil  in  gaining  a  livelier  conception 
of  the  phenomena  treated  in  the  book.  Small  wooden  or  metal  models 
of  essential  machine  parts  and  instruments  not  used  in  the  laboratory 
may  add  an  element  of  life  and  interest  to  class-room  instruction,  as 
well  as  conduce  to  clearer  conceptions  on  the  part  of  the  pupils.  When 
the  time  for  class-room  instruction  is  limited,  well-informed  teachers 


PREFACE  Vll 

may  readily  select  the  portions  of  the  book  that  are  most  suitable  for 
their  purpose. 

The  proof  of  the  book  was  twice  read  by  Principal  G.  W.  Krall,  of 
the  St.  Louis  (Missouri)  Manual  Training  School,  and  Superintendent 
F.  A.  Lowell,  of  the  Rhinelander  (Wisconsin)  Schools ;  and  a  part  of  it 
was  read  by  Assistant  Professor  Fred  A.  Fish,  of  the  Ohio  State  Uni- 
versity. The  authors  are  in  their  debt  for  numerous  suggestions  which 
have  added  to  the  clearness  and  accuracy  of  the  treatment.  Professor 
A.  C.  Scott,  of  the  Rhode  Island  College  of  Agriculture  and  Mechanic 
Arts  (Honorary  Fellow  in  Electrical  Engineering  at  the  University  of 
Wisconsin),  lent  efficient  aid  while  the  book  was  passing  through  the 
press,  and  also  made  the  photographs  from  which  various  illustrations 
were  produced.  So  much  care  has  been  given  to  the  preparation  of  the 
manuscript  and  the  reading  of  the  proofs  that  it  may  be  justly  hoped 
that  any  remaining  imperfections  are  of  only  minor  character.  The 
writers  will  be  grateful  to  teachers  and  others  for  notice  of  inaccuracies 
or  obscure  passages  which  may  be  discovered. 

THE    AUTHORS. 


. 


TABLE    OF   CONTENTS 

CHAPTER   I 

PAGE 

The  Nature  and  Properties  of  Electricity i     L 

CHAPTER    II 
Additional  Characteristics  of  Electric  Charges 10 

CHAPTER    III 
Electrical  Potential,  Electrical  Machines,  and  Electrical  Capacity  .         .       16 

CHAPTER    IV 

Electric  Batteries,  or  Appliances  for  Transforming  Chemical  Energy 

into  Electrical  Energy 28 

CHAPTER    V 
Electrolysis   .         .         .         .- 51 

CHAPTER   VI 

The  Nature  and  Properties  of  Magnetism 63     v 

CHAPTER    VII 

Electric  Circuits  and  the  Flow  of  Electricity  ;  Ohm's  Law      ...       82 

ix 


X  TABLE   OF   CONTENTS 

CHAPTER   VIII 

PAGE 

Electrical  Energy,  Heating  Effects  of  Electric  Currents,  and  Miscel- 
laneous Effects  of  Electric  Currents 104 

CHAPTER   IX 
I/  Electro-magnetism  .  .  .119 

CHAPTER   X 

Electro-magnetic  Induction    .  .  .  .     138 

CHAPTER   XI 
Galvanometers  and  Voltameters .     156 

CHAPTER   XII 
Measurement  of  Electrical  Resistance     .         .         .         .         .         .         .169 

CHAPTER   XIII 
Measurement  of  Electric  Currents  and  Pressures 182 

CHAPTER   XIV 

Measurement  of  Electrical  Power.     Condensers,  and  Measurement  of 

Capacity          ...........     200 

CHAPTER   XV 

Principles  and  Construction  of  Direct  Current  Dynamos  and  Motors      .     212     , 

CHAPTER   XVI 

Alternating  Currents  and  Alternating  Current  Machinery        .         .         .     236 


TABLE   OF   CONTENTS  xi 

CHAPTER    XVII 


PAGE 


Arc  and  Incandescent  Lighting      .  273 

CHAPTER   XVIII 
Power  Stations,  the  Electric  Railway,  and  Other  Applications  of  Motors     291 

CHAPTER   XIX 
The  Telegraph  ;  the  Telephone  ;  Electric  Bells      .      £j  ^\X"-^~    .     335 

CHAPTER   XX 

Line  Construction  and  the  Electric  Distribution  and  Transmission  of 

Power 369 

CHAPTER   XXI 

Applications  of  Electrical  Instruments  to  the  Testing  of  Lines  and  Cir- 
cuits.    Measurements  of  Illumination      ......     410 

CHAPTER  XXII 

Electrolytic  Deposition  of  Metals.     Electric  Smelting,  Welding,  Cook- 
ing, etc.  I  .430 

CHAPTER   XXIII 
Electro-magnetic  Waves  ;  Wireless  Telegraphy  ;  Roentgen  Rays  .         .     455 

INDEX 469 


ELECTRICITY   AND    MAGNETISM 

CHAPTER   I 

THE  NATURE  AND   PROPERTIES   OF  ELECTRICITY 

1.  Electricity.  —  The  exact  nature  of  the  electricity  which  makes 
itself  evident  in  so  many  ways  has  never  been  determined.  Many  sur- 
mises or  theories  have  been  advanced,  but  none  have  yet  been  able  to 
fully  stand  the  test  of  close  examination.  But  by  experimental  evidence 
(which  has  been  gathered  for  decades)  we  have  been  able  to  determine 
some  of  the  laws  which  govern  the  action  of  electricity,  though  we  do 
not  know  its  constitution,  very  much  as  we  have  learned  the  laws  of 
gravitation,  though  we  do  not  know  what  "  gravity  "  really  is. 

The  etymology  and  use  of  the  word  "  electricity  "  have  developed 
in  parallel  with  the  experimental  growth  of  the  science  which  bears  its 
name.  Springing  from  the  Latin  name  for  amber,  electricus  or  elec- 
trum,  the  adjective  Electrical  comes  immediately  from  the  word  "elec- 
tric," which  was  used  in  a  book  published  in  1600  by  Dr.  Gilbert  (the  great 
scientist  of  Queen  Elizabeth's  reign),  to  designate  the  attraction  for  light 
bodies  like  chaff  and  bits  of  paper  which  amber  and  similar  substances 
exhibit  when  briskly  rubbed.  The  original  discovery  of  this  electrical 
property  (or  property  of  the  amber)  is  often  attributed  to  a  Greek 
philosopher  (one  of  the  "  seven  wise  men  "  of  Greece)  named  Thales, 
who  lived  about  600  years  before  the  Christian  era,  and  the  meagre 
reports  of  whose  philosophy  are  thought  by  some  to  contain  the  earliest 
records  of  its  observation  that  have  come  down  to  us.  It  is  probable, 
however,  that  a  knowledge  of  this  peculiar  property  of  amber,  and 
possibly  of  other  bodies,  was  one  of  the  well-guarded  secrets  of  the 
priesthood  of  that  day. 


2  ELECTRICITY   AND    MAGNETISM 

From  the  word  "  electric  "  also  comes  the  word  Electricity.  Since  the 
day  Dr.  Gilbert  first  applied  the  word  "  electric  "  to  a  particular  phe- 
nomenon, our  knowledge  of  all  the  sciences  has  widened,  and  with  the 
widening  has  come  an  equal  advance  in  the  knowledge  which  was 
represented  to  the  ancients  by  that  one  peculiar  property  of  amber 
and  similar  bodies.  The  term  "  electricity  "  is,  therefore,  not  now  ap- 
plied to  only  one  small  branch  of  a  great  science,  but  covers  a  vast 
field  of  facts  which  are  supposed  to  be  based  on  the  same  underlying 
causes. 

2.  The  Nature  of  Electricity.  —  The  action  of  electricity  led  many 
experimenters  who  lived  long  after  Gilbert  to  the  belief  that  it  was  a 
fluid  which  was  not  perceptible  to  their  senses.  Our  own  great  philoso- 
pher and  statesman,  Benjamin  Franklin,  assumed  it  to  be  a  fluid,  and 
bodies  which  exhibited  electrical  manifestations  were  thought  by  him 
to  contain  either  more  or  less  than  a  normal  amount  of  the  fluid.  A 
Frenchman  named  Dufay  and  an  Englishman  named  Symmer  con- 
sidered electricity  to  be  composed  of  two  fluids  which  were  contained 
in  neutral  bodies  in  equal  amounts.  When  by  any  means  this  equality 
was  disturbed  in  a  body,  electrical  manifestations  occurred. 

These  theories,  and  a  large  number  similar  to  them  that  were  pro- 
mulgated, are  now  discarded  in  the  light  of  later  scientific  knowledge. 
But  the  conception  of  the  fluid  theory  is  very  useful  in  giving  a  clear 
understanding  of  some  of  the  phenomena  of  electricity.  It  is  now 
generally  accepted  that  the  phenomena  to  which  we  give  the  name 
electricity  result  from  a  state  of  strain  or  other  manifestation  in  the 
Ether.  The  ether  is  a  kind  of  fluid  medium  that  is  supposed  by  scien- 
tists to  be  present  everywhere.  It  must  even  be  supposed  to  pass 
through  or  be  contained  in  solid  bodies,  as  though  they  were  ether  sieves, 
as  well  as  in  empty  space.  Heat  and  light  are  supposed  to  be  carried 
by  it  from  one  body  to  another,  as  from  the  sun  to  the  earth,  by  means 
of  vibrations  or  waves,  much  as  the  energy  exerted  by  a  pebble  thrown 
into  a  pond  is  carried  to  the  shores  by  the  waves  of  the  water.  In  like 
manner  electricity  is  supposed  to  be  waves  in  the  ether  or  a  strain 
imposed  on  it.  The  question  of  what  the  ether  may  really  be  need  not 
be  considered  in  dealing  with  the  fundamental  laws  governing  the  action 
of  electricity. 


THE  NATURE  AND    PROPERTIES   OF   ELECTRICITY  3 

3.  Static  and  Current  Electricity.  —  We  will  not  at  this  time  further 
discuss  the  nature  of  electricity,  but  will  pass  on  to  a  consideration  of 
its  properties.     The  study  of  these  properties  may  be  suitably  divided 
into  two  classes  —  the  first,  in  which  Static  Electricity,  or  electricity  at 
rest,  is  considered;  and  the  second,  in  which  Current  Electricity,  or 
electricity  in  motion,  is  considered.     There  is  no  well-defined  division 
between  these,  and  the  laws  governing  the  two  classes  are  practically 
the  same. 

In  general,  however,  the  first  class  includes  the  phenomena  known 
by  the  ancients,  where  electricity  is  produced  by  rubbing  or  by  the  in- 
fluence of  one  Electrified  body  on  another.  The  second  class  includes 
electricity  produced  by  the  electric  batteries  and  dynamos  which  are  so 
well  known  to-day.  The  first  class  is  of  comparatively  small  importance 
and  will  receive  only  such  attention  in  the  earlier  part  of  this  book  as 
is  necessary  on  account  of  its  bearing  on  the  second  class. 

4.  Positive   and    Negative   Electricity.  —  If  a   rod   of   sealing-wax, 
amber,  or  other  resinous   substance   is  rubbed  with  a  dry  wool  cloth 
or  fur,  it  immediately  gains  the  property  of  attracting  to  itself  light 
bodies,  such  as  pith  or  bits  of  paper.     After  the  bits  of  pith  have  been 
in  contact  with  the  rubbed  body  for  a  short  time,  they  usually  fly  off 
as  if  repelled,  and  they  also  seem  to  repel  each  other.     The   rubbed 
body  when  in  the  condition  produced  by  the  rubbing  may  be  found  by 
proper  examination  to  be  covered  with  an  apparent  layer  of  electricity, 
which  is  called  a  Charge,  and  the  body  is  said  to  be  Charged  with  elec- 
tricity.    The  pith  balls  which  touch  the  rubbed  body  become  covered 
with  a  similar  layer,  and  are  also  said  to  be  charged. 

If  a  glass  rod  is  rubbed  with  silk,  it  will  show  properties  like  those 
of  the  resinous .  substances.  But,  if  the  glass  rod  is  brought  close  to 
the  pith  balls  which  have  been  in  contact  with  and  were  repelled  by 
the  resin  rod,  it  will  strongly  attract  them ;  and  in  the  same  way  the 
resinous  rod  attracts  pith  balls  which  have  been  charged  by  contact  with 
the  glass. 

These  experiments  seem  to  prove  the  existence  of  two  kinds  of  elec- 
tricity, which  are  called  vitreous  or  Positive  electricity,  and  resinous  or 
Negative  electricity,  depending  on  whether  they  are  produced  by  rubbing 
glass  with  silk,  or  resinous  materials  with  wool.  The  action  of  the  pith 


4  ELECTRICITY  AND    MAGNETISM 

balls  also  shows  'that  bodies  charged  with  one  kind  of  electricity  repel 
those  charged  with  the  same  kind,  but  attract  those  charged  with  the  oppo- 
site kind.  Charged  bodies  are  also  said  to  be  Excited  or  Electrified. 

Other  similar  manifestations  of  electricity  may  be  easily  produced. 
For  instance,  if  a  well-dried  sheet  of  paper  is  laid  on  a  table  and  briskly 
rubbed  with  a  rubber  eraser  or  a  coat  sleeve,  it  will  -seem  to  stick  to  the 
table ;  and  when  it  is  slowly  raised  by  one  corner,  small  sparks  may  be 
seen  to  pass  between  it  and  the  table,  if  the  room  is  dark. 

On  dry  days  it  is  sometimes  possible  for  a  person  to  gather  a  charge 
on  his  body  by  shuffling  his  feet  across  the  carpet.  This  charge  may 
be  sufficient  to  produce  a  spark  if  the  finger  is  presented  to  a  gas  fixture 
or  to  another  person. 

Again,  if  a  charged  body  is  held  near  to  the  face,  a  peculiar,  tickling 
sensation  may  be  felt  on  account  of  the  attraction  of  the  small  hairs  on 
the  cheeks  by  the  charge. 

5.  Positive  Charges  develop  Equal  Negative  Charges,  and  vice  versa.  — 
If  the  wool  used  to  develop  a  negative  charge  on  the  sealing-wax  by  rub- 
bing is  now  tested  by  bringing  a  charged  pith  ball  near  to  it,  it  also  is 
found  to  be  charged,  —  this  charge  being  positive.     In  the  same  way 
the  silk  which  was  used  in  rubbing  the  glass  may  be  found  to  be  nega- 
tively charged,  the  charge  on  the  glass  having  been  positive. 

These  observations  are  in  accordance  with  an  important  fact  which  has 
been  experimentally  proved  :  that  whenever  a  charge  of  one  kind  is 
developed,  an  equal  charge  of  the  opposite  kind  is  also  developed. 

6.  The  Character  of  the  Charge  on  Rubber  and  Rubbed.  —  When  two 
dry  bodies  of  different  materials  which  do  not  have  the  power  of  con- 
ducting electricity  are  rubbed  together,  they  always  become    charged 
with  opposite  kinds  of  electricity.     If  one  of  these  bodies  is  then  rubbed 
with  a  third  material,  its  charge  may  be  changed.     The  kind  of  charge 
which  appears  on  the  body  of  one  material  when  rubbed  with  another 
material  depends  altogether  on  the  nature  of  the  two  materials  that  are 
rubbed  together.     For  instance,  as  we  have  seen,  when  glass  is  rubbed 
with  silk  the  glass  becomes  positively  charged,  and  the  silk  negatively 
charged.     If  a  stick  of  sulphur  is  rubbed  with  silk,  the  order  is  reversed 
and  the  silk  becomes  positively  charged,  while  the  sulphur  is  negatively 
charged. 


THE  NATURE  AND    PROPERTIES   OF   ELECTRICITY  5 

Following  out  an  investigation  of  this  kind,  it  is  possible  to  arrange  a 
table  of  materials  in  which  they  are  placed  in  such  an  order  that  when 
any  two  materials  named  in  the  table  are  rubbed  together,  the  one  that 
stands  earliest  in  the  table  will  ordinarily  become  positively  charged  and 
the  other  negatively  charged.  The  following  table  is  so  arranged.  Its 
correctness  may  be  easily  tested  by  experiments. 

1.  Fur.  7.   Wood. 

2.  Wool.  8.   Metals. 

3.  Some  Resinous  Substances.  9.   Sulphur. 

4.  Glass.  10.   Other  Resinous  Substances. 

5.  Cotton.  ii.    India-rubber. 

6.  Silk.  12.    Gutta-percha. 

The  reason  for  this  difference  in  materials  is  not  known,  and,  in  fact, 
slight  differences  in  the  constitution  or  the  surface  of  the  materials  may 
cause  them  to  change  their  relative  positions,  so  that  similar  tables  given 
in  various  books  do  not  all  agree. 

7.  Conductors  and  Insulators.  —  If  a  piece  of  metal  is  held  in  the 
hand  and  rubbed,  no  apparent  charge  can  be  discovered  on  it.  This  is 
because  the  metal  has  the  power  of  readily  conducting  electricity,  as  it 
has  likewise  the  power,  of  conducting  heat,  and  the  electricity  therefore 
all  flows  away  into  the  body  of  the  operator,  or  through  his  body  into 
the  earth.  The  same  thing  is  true  of  any  of  the  substances  named  in  the 
table  if  they  are  dampened  with  water,  because  water  has  the  power,  to 
a  limited  degree,  of  conducting  electricity.  Consequently,  experiments 
with  static  electricity  cannot  be  readily  made  on  a  damp  day  or  when 
the  materials  are  damp. 

If  the  metal  is  fastened  in  a  handle  of  dry  wood  or  hard  rubber  and 
again  rubbed,  it  will  then  become  charged.  This  is  because  the  wood  or 
hard  rubber  does  not  have  the  power  of  conducting  the  electricity  to  an 
extent  which  is  in  this  case  appreciable,  and  the  charge  of  electricity, 
therefore,  cannot  escape,  but  remains  on  the  metal. 

Materials  which  readily  conduct  electricity  are  called  Conductors,  and 
those  which  either  do  not  conduct  it  at  all  or  only  conduct  it  in  a  very 
small  degree  are  called  Non-conductors  or  Insulators.  Other  materials 


6  ELECTRICITY  AND   MAGNETISM 

which  have  the  conducting  power  in  an  intermediate  degree  are  often 
called  Partial  Conductors. 

The  following  table  gives  a  list  of  materials  placed  approximately  in 
the  order  of  their  conducting  powers  :  — 

1.  Metals.  7.  Various  Oils.  13.  Vulcanite. 

2.  Charcoal  and  Graphite.  8.  Dry  Wood.  14.  Paraffine. 

3.  Acids.  9.  Silk.  15.  Porcelain. 

4.  Salty  Solutions.  10.  India-rubber.  16.  Glass. 

5.  Plants  and  Animals.  n.  Mica.  17.  Dry  Air. 

6.  Pure  Water.  12.  Shellac. 

We  ordinarily  restrict  the  term  "  conductor  "  to  the  metals.  The  materi- 
als in  the  table  numbered  from  two  to  six  may  be  called  "  partial  con- 
ductors," and  the  last  eleven  materials  may  be  called  "  insulators."  Of  all 
the  materials  named,  dry  air  may  be  said  to  be  the  only  one  which  has 
absolutely  no  conducting  power  under  ordinary  conditions,  though  that 
of  glass,  porcelain,  etc.,  is  exceedingly  small  at  ordinary  temperatures. 

The  cause  of  the  difference  in  the  conducting  powers  of  the  various 
materials  is  not  known,  and  will  probably  not  be  known  until  the  exact 
constitution  of  electricity  is  determined.  When  science  succeeds  in 
unravelling  that  mystery,  we  will  probably  also  learn  the  exact  nature  of 
light,  and  the  cause  of  the  attraction  of  gravitation,  together  with  the 
reasons  for  many  other  highly  important  laws  the  explanations  of  which 
are  now  held  among  the  profound  secrets  of  nature. 

By  means  of  the  great  conducting  power,  or  Conductivity,  of  metals, 
electricity  may  be  conveyed  from  place  to  place.  If,  for  instance,  two 
blocks  of  metal  connected  by  a  wire  are  mounted  on  insulators,  and  if 
a  charge  is  given  to  one,  a  part  of  the  electricity  will  flow  along  the  wire  to 
the  second  block  and  will  electrify  it.  A  conductor  which  is  supported 
on  insulators  in  such  a  way  that  electricity  cannot  escape  from  it  is  said 
to  be  Insulated. 

8.  Induced  Charges.  —  A  body  may  also  be  charged  or  electrified  by 
the  influence  upon  it  of  a  charged  body.  Thus,  suppose  a  brass  ball  is 
insulated  and  charged,  and  that  it  is  brought  near  an  uncharged  but 
insulated  brass  ball.  The  second  ball  will  now  be  found  to  be  charged, 
if  it  is  tested  by  bringing  a  charged  pith  ball  near  to  it.  A  charge  which 


THE  NATURE  AND   PROPERTIES  OF   ELECTRICITY 


is  developed  in  this  manner  by  the  influence  of  a  charged  body  on  a 
Neutral  or  uncharged  one,  is  said  to  be  developed  by  Induction. 

If  the  brass  ball  on  which  the  charge  is  thus  Induced  is  carefully  exam- 
ined, its  two  sides  will  be  found  to  hold  opposite  kinds  of  electricity 
(Fig.  i).  The  side  of  the  second  ball  which  is 
away  from  the  first  ball  will  hold  the  same  kind 
of  electricity  as  the  latter,  and  the  side  which  is 
near  the  first  ball  will  hold  the  opposite  kind. 
This  is  in  accordance  with  the  law  of  attraction 
and  repulsion  between  the  different  kinds  of 
electricity  given  in  Article  4.  For  example,  if 
the  first  ball  (A  in  Fig.  i )  is  positively  charged, 
the  side  of  the  second  ball  (B  in  Fig.  i)  which 
is  away  from  the  first  will  also  be  positively 
charged,  but  the  near  side  will  be  negatively 
charged.  This  is  the  condition  shown  in  the 
figure,  where  the  plus  or  positive  sign,  +,  rep- 
resents a  positive  charge,  and  the  minus  or  neg- 
ative  sign,  — ,  represents  a  negative  charge. 

Now,  if  the  second  ball  is  touched  by  one's 
finger  for  an  instant  when  it  is  very  close  to  the 
first,  the  positive  charge  will  immediately  flow 
away  into  the  operator's  body  on  account  of   FIG.  i.  — Illustration  of  Elec- 
the  repulsion  between  the  positive  charge  on          trostatic  induction. 

the  second  ball  and  that  which  is  on  the  first  Two  brass  balls,  ,4  and  2?,  sus- 
.     „       „,  11,1  pended  by  silk  insulating 

ball.     The  negative  charge  on  the  second  ball         cords. 
will  remain  on  account  of  the  attraction  be- 
tween it  and  the  charge  on  the  first  ball.     If  the  second  ball  is  now 
removed  from  the  influence  of  the  first  ball,  it  will  remain  negatively 
charged,  the  charge  spreading  all  over  it.     And  if  the  two  balls  are  now 
brought  into  contact  with  each  other,  the  two  charges  will  combine  and 
both  balls  will  become  Neutral,  that  is,  without  any  charge. 

The  experiment  described  in  the  last  paragraph  shows  that  each  induced 
charge  is  equal  in  quantity  to  the  charge  which  induces  it.  This  fact  is 
strictly  true,  but  in  many  cases  the  induced  charge  is  divided  among 
several  objects  which  are  near  a  charged  body,  so  that  it  may  be  difficult 


ELECTRICITY  AND    MAGNETISM 

to  get  complete  neutralization  in  the  manner  explained.  The  induced 
charges  are  to  be  found  wholly  on  one  body  only  when  it  completely  sur- 
rounds the  charged  one ;  but  when  a  body  is  very  much  nearer  to  the 
charged  one  than  any  other  bodies,  it  will  receive  practically  the  whole 
charge. 

The  object  in  using  brass  balls  in  such  experiments  is  simply  to  obtain 
convenient  and  inexpensive  conductors.  Any  other  materials  will  give 
similar  results,  but  in  the  case  of  poorly  conducting  bodies  it  is  more 
difficult  to  perceive  the  results  on  account  of  the  difficulty  which  such 
bodies  present  to  the  distribution  of  electricity  under  the  influence 
of  induction. 

QUESTIONS 

1.  How  much  is  known  about  the  real  constitution  of  electricity? 

2.  What  is  the  origin  of  the  word  "  electricity  "  ? 

3.  What  is  electricity  supposed  to  be  by  some  scientists? 

4.  Is  there  any  difference  between  current  and  static  electricity? 

5.  What  two  manifestations    of    electricity  are    produced  when   materials   are 
rubbed? 

6.  How  can  the  presence  of  a  charge  of  electricity  be  shown? 

7.  Can  a  charge  of  one  kind  of  electricity  exist  alone? 

8.  What  effect  have  unlike  charges  of  electricity  upon  each  other? 

9.  Do  like  charges  repel  or  attract? 

10.  If  a  positive  charge  is  developed  by  rubbing,  how  large,  relatively,  will  the 
accompanying  negative  charge  be? 

11.  If  glass  is  rubbed,  will  it  always  take  a  positive  charge,  regardless  of  the 
material  of  the  rubber? 

12.  If  gutta-percha  is  rubbed  by  fur,  which  will  take  a  positive  charge? 

13.  What  precautions  must  be  taken  in  handling  metals  in  order  that  they  may  be 
charged  by  rubbing? 

14.  What  is  a  conductor? 

15.  What  is  an  insulator? 

16.  What  is  the  meaning  of  electrical  conductivity? 

17.  Name  some  good  conductors. 

1 8.  Name  some  partial  conductors. 

19.  Name  some  ordinary  materials  that  are  good  insulators. 

20.  How  may  electricity  be  conveyed  from  one  place  to  another? 

21.  What  effect  does  a  charged  conductor  have  upon  an  uncharged  conductor 
which  is  brought  near  it  ? 

22.  What  is  the  meaning  of  electric  induction? 


THE  NATURE   AND   PROPERTIES   OF   ELECTRICITY  9 

23.  How  can  an  induced  charge  be  kept  on  a  body  when  it  is  removed  from  the 
neighborhood  of  the  body  that  induced  the  charge? 

24.  How  will  the  quantity  of  an  induced  charge  compare  with  the  quantity  of 
the  charge  on  the  inducing  body? 

.15.    What  kind  of  electricity  will  a  positively  charged  ball  induce? 

26.  If  an  uncharged  ball  is  held  near  a  negatively  charged  one,  what  kind  of  elec- 
tricity will  be  evident  on  the  part  of  the  first  ball  most  distant  from  the  charged  ball? 

27.  If  a  charged  body  is  held  in  the  middle  of  an  otherwise  vacant  room,  where 
will  the  induced  charges  be  found? 


CHAPTER   II 

ADDITIONAL  CHARACTERISTICS  OF 'ELECTRIC  CHARGES 

9.  Electroscopes.  —  The  means  of  detecting  a  charge  thus  far  men- 
tioned have  been  through  the  attraction  or  repulsion  of  charged  pith 
balls  or  other  light  objects.  Various  other  means  may  be  used,  all  of 
which  are  dependent  upon  electrical  attractions  and  repulsions  for  their 
indications. 

Devices  or  instruments  for  determining  the  presence  of  an  electric 
charge  are  called  Electroscopes.  The  simplest  one  consists  of  a  pith 
ball  (preferably  gilded)  or  other  light  material, 
attached  to  a  silk  insulating  suspending  cord. 
A  more  sensitive  one  is  made  by  attaching  two 
narrow  strips  of  ordinary  gold  leaf,  such  as  can 
be  obtained  from  any  dentist,  to  the  end  of  a 
brass  rod,  and  hanging  the  leaves  in  a  glass 
bottle  to  insulate  them  and  protect  them  from 
injury  (Fig.  2).  If  a  charged  body  is  brought 
near  the  top  of  the  rod  which  is  connected  to 
the  gold  leaves,  the  rod  and  leaves  are  elec- 
trified by  induction.  If  the  charged  body  is  a 
rubbed  glass  rod  which  is  positively  charged,  as 
in  Figure  3,  a  negative  charge  will  appear  at  the 
top  of  the  conductor  and  a  positive  one  in  the 
gold  leaves.1  In  this  case,  since  the  two  leaves 
have  charges  of  the  same  kind,  they  will  repel 
each  other  and  separate,  as  is  illustrated  in  the  figure.  The  gold  leaves 
are  so  sensitive  that  they  are  likely  to  be  torn  by  the  force  of  their 
repulsion  if  a  heavily  charged  body  is  brought  too  close. 

1  Compare  the  case  of  the  brass  balls  given  in  Article  8.  t 

10 


GOLD 
LEAVES 


FIG.  2.  —  Gold-leaf  Electro- 
scope. 


AODITIONAL   CHARACTERISTICS   OF   ELECTRIC   CHARGES        II 


If,  while  the  glass  rod  is  still  held  near  the  electroscope,  the  brass  rod 
of  the  electroscope  is  touched  by  the  hand,  the  positive  charge  in  the 
leaves  will  at  once  flow  off  into  the  operator's  body  on  account  of  the 
repulsion  of  the  charge  on  the  glass  (as  was  explained  in  the  description 
of  the  experiment  of  the  brass  balls  given 
above),  and  the  leaves  will  drop  together. 
Now,  if  the  glass  rod  is  taken  away,  the 
negative  charge  which  was  retained  by  the 
attraction  of  the  charge  on  the  rod  will 
spread  all  over  the  electroscope  rod  and 
gold  leaves,  and  the  leaves  will  again  sep- 
arate. It  can  be  easily  proved  that  the 
charge  on  the  gold  leaves  is  now  negative 
by  bringing  the  positively  charged  glass 
rod  near  the  top  of  the  electroscope,  when 
the  negative  charge  will  be  attracted  out 
of  the  leaves  and  they  will  fall  together. 
Or.  if  a  negatively  charged  rod  of  sealing- 
wax  is  brought  near  the  top  of  the  electro- 
scope, the  negative  charge  in  the  instrument 
may  all  be  repelled  into  the  leaves  and  they 
will  separate  farther.  With  this  simple  de- 
vice it  is  possible  to  detect  a  very  small  charge  of  electricity  and  deter- 
mine its  sign. 

The  electroscope  may  of  course  be  directly  charged  by  contact  with 
a  charged  body,  but  the  leaves  are  likely  to  be  torn  by  the  violence  of 
the  action,  unless  the  charge  is  quite  small. 

10.   Reason  for  Attraction  between  Charged  and  Light  Bodies.  —  We 

are  now  in  position  to  see  the  reason  for  the  attraction  which  rubbed 

amber,  rubbed  glass,  and  other  charged  bodies  have  for  light  objects. 

Since  electric  induction  acts  between  any  charged  body  and  any  other 

body  which  is  reasonably  near,  the  effect  of  the  charged  body  on  a  light 

object  is  first  to   charge  it  by  induction.     The  positive    and  negative 

induced  on  the  light  object  are  equal  in  quantity.     One  of  them 

'tracted  and  the  other  is  repelled   by  the   original   charge.     That 

which  is  attracted  is  nearer  the  original  charge,  so  that  the  force  of 


FIG.  3.  —  Gold-leaf  Electroscope 
charged  by  Induction. 


12 


ELECTRICITY  AND   MAGNETISM 


-fa 


attraction  is  greater  than  the  force  of  repulsion.  The  condition  is  illus- 
trated in  Figure  i,  which  is  here  repeated.  A  positive  charge  is  seen  at 
a  on  the  large  ball.  This  induces  the  negative  , 
and  positive  charges  b  and  c  on  the  small  ball. 
Since  b  is  considerably  nearer  a  than  is  c,  the 
attraction  between  a  and  b  is  materially  greatt  r 
than  the  repulsion  between  a  and  c.  The  small 
ball  is  therefore  attracted  toward  the  large  b 
If  the  balls  come  in  contact,  the  small  ball  re- 
ceives a  part  of  the  positive  charge  belonging 
to  the  large  one,  and  they  at  once  separate  on 
account  of  the  repulsion  between  the  two  posi- 
tive charges. 

The  attraction  or  repulsion  between  a  char 
body  and  one  which  it  charges  by  induct 
or  between  two  independently  charged  o 
always  exists,  though  the  pull  or  push  exerted 
by  the  charges  usually  is  sufficient  to  move 
bodies  only  when  they  are  very  light. 

11.    Force  of  Attraction  or  Repulsion.  — ' 
FIG.  i.  —  illustration  of  Elec-  intensity  of  the  attraction  or  repulsion  exei 
trostatic  induction.          between  any  two  charged  bodies  depends  u 

Two  brass  balls,  A  and  B,  sus-  the  product  of  the  quantities  of  electricity  in  t 
pended  by  silk  insulating      , 
cords>  charges,  their  distance  apart,  and  the  matt '  uil 

which  is  between  them.    If  they  are  surroun 

by  air,  the  push  or  pull  which  is  exerted  between  the  two  bodies  incrc 
directly  with  the  product  of  the  quantities  of  electricity  which  they  / 
and  decreases  directly  with  the  square  of  the  distance  between  them,  pro- 
vided the  bodies  are  small  compared  with  that  distance.     If  the  two 
charged  bodies  are  immersed  in  a  liquid,  SK  '-  .  as  water  or  oil,  or  are 
separated  by  solids,  the  same  law  holds  true,  t,      the  force  exerted  be- 
tween them  is  decreased.     The  amount  of     .^.decrease  depends  upon 
the  nature  of  the  separating  material,  and  is          x  ntly  due  to  a  difficulty 
met  by  the  attractive  force  in  making  its  wa,  tnrou^h  the  material. 
12.    The    Coulomb.  —  The   unit   quantity   of  electricity   is   call* 
Coulomb,  after  a  French  experimenter  who  lived  about  the  beginning  of 


O 

B 


ADDITIONAL  CHARACTERISTICS  OF   ELECTRIC  CHARGES        13 

the  nineteenth  century.  As  a  rough  analogy  with  the  measurement  of 
water  or  gas,  we  may  say  that  a  coulomb  of  electricity  is  the  equivalent 
of  a  gallon  of  water  or  a  cubic  foot  of  gas. 

The  reason  that  the  force  exerted  between  two  charged  bodies  de- 
pends upon  the  product  of  the  two  quantities  of  electricity,  is  that  each 
lomb  of  electricity  on  one  body  attracts  or  repels  every  coulomb  on 
the  other  body  with  a  fixed  intensity,  and  therefore  the  total  force  of 
attraction  or  repulsion  depends  on  the  number  of  coulombs  on  one 
body  multiplied  by  the  number  on  the  other  body. 

The  intensity  in  pounds  of  the  push  or  pull  exerted  between  two 
charged  bodies  in  air,  which  has  been  described  in  italics  in  Article  n, 
may  be  represented  algebraically  by  the  expression 

12  ^    X    4 

20  •  2  x  io12  x        2    , 

if  q  and  q1  are  understood  to  represent  the  number  of  coulombs  in  the 
respective  charges  of  electricity,  and  d  is  understood  to  represent  the 
distance  between  the  bodies  measured  in  centimeters.  The  expression 
io12  stands  for  one  trillion  which  must  be  multiplied  into  the  product. 
(A  centimeter  is  a  metric  measure  of  length  which  is  equal  to  about  .39 
of  an  inch.) 

Example. — Two  similarly  charged  small  bodies  are  at  a  distance 
apart  of  6  centimeters.  One  carries  a  charge  of  6  ten-millionths  of  a 
coulomb,  and  the  other  carries  a  charge  of  9  ten-millionths  of  a  coulomb. 
What  is  the  intensity  of  the  repulsion  exerted  between  them?  Ans. 
.  03  pounds. 

13.  Electrometers.  —  Instruments   for   determining   the   quantity  of 
electricity  which  is  held  in  charge  on  a  body,  by  measuring  its  attraction 
for  another  charged  body,  are  called  Electrometers.     These  instruments 
are  valuable  for  many  pur-       s,  and  will  receive  more  attention  in  later 
chapters.     It  is  to  be  bon     m  mind  that  electroscopes  are  instruments 
for  detecting  a  charge,  and  t%:  ,;trometers  are  instruments  for  measuring 
the  quantity  of  electricity  harge. 

14.  Static  Electricity  tends  to  stay  on  the  Surface  of  a  Conductor.  — 
One  of  the  peculiar  properties  of  electricity  results  in  a  charge  always 
locating  on  the  surface  of  a  charged  conductor.     We  often  hear  the 


ELECTRICITY  AND   MAGNETISM 


statement  made  "  that  electricity  flows  only  on  the  surface  of  a  wire." 
This  is  entirely  untrue.  When  electricity  flows  or.  moves  it  apparently 
passes  through  the  substance  of  the  conductor.  In  the  case  of  electricity 
at  rest,  however,  the  condition  is  different.  When  electricity  is  at  rest  it 
never  remains  in  the  substance  of  a  body,  but  stays  strictly  on  the  surface. 
It  is  important  that  this  difference,  described  in  the  last  paragraph, 
between  the  action  of  electricity  in  motion,  or  current  electricity,  and 
electricity  at  rest,  or  static  electricity,  shall  be  remembered. 

Again,  a  free  charge  of  static  electricity  not  only  stays  on  the  surface 
of  a  body,  but  it  tends  to  stay  on  the  outside  surface.     Figure  4  shows 
•         .  this  by  the  position  of  the  pith  balls,  which  are 

\/  suspended  on  the  inside  and  outside  of  a  hol- 

low brass  cylinder.  The  cylinder  being  freely 
charged,  the  outside  pith  balls,  which  are  in 
contact  with  it,  at  once  diverge  on  account  of 
a  charge  which  they  receive  from  the  cylinder. 
This  shows  that  the  outer  surface  of  the  cylinder 
is  charged.  The  inner  pith  balls,  which  are  also 
in  contact  with  the  cylinder,  remain  entirely 
inert,  showing  that  there  is  no  charge  on  the 
inner  surface.  This  is  true  whether  the  charge 
is  given  to  the  cylinder  from  the  inside  or  out- 
side, and  is  to  be  expected  on  account  of  the 
known  repulsion  of  like  charges  or  parts  of  a 
charge.  The  different  parts  of  a  free  charge 
try  to  get  as  far  ,away  from  each  other  a^ 
sible,  and  therefore  go  to  the  outer  surface  of  a 
body  if  its  conductivity  is  sufficient  to  permit  it. 
A  brass  cylinder  is  used  in  this  experiment  so  that 
the  electricity  may  readily  follow  its  tendency  to 
move  to  the  outer  surface  if  it  should  be  applied  at  the  inner  sun 

It  is  possible   to  retain  an   induced   charge   on  the  interior  of  the 
cylinder  by  placing  and  keeping  a  charged  ball  inside  of  the  cylinder. 
But  an  equal  and  opposite  induced  charge  also  appears  at  the  sa 
on  the  outside  of  the  cylinder.      If  the  charged  ball  is  removed,  the 
induced  charges  disappear;    and  if  the  charged  ball  is  pern 


FIG.  4.— Hollow  Cylinder 
with  Electrostatic  Charge. 


ADDITIONAL   CHARACTERISTICS   OF   ELECTRIC  CHARGES        15 

touch  the  inside  of  the  cylinder,  the  induced  charges  disappear,  and  all 
of  the  charge  on  the  ball  at  once  goes  to  the  outside  of  the  cylinder. 

15.  Electric  Screens.  —  By  virtue  of  the  fact  that  a  charge  tends  to 
stay  on  the  outer  surface  of  a  body,  it  is  possible  to  entirely  screen  an 
object  from,  all  electrostatic  force  by  completely  surrounding  it  with  a 
conducting  cage.  This  is  done  in  making  electrometers,  when  it  is 
desirable  to  screen  the  working  parts  of  the  instrument  from  the  outside 

electric  forces. 

QUESTIONS 

1.  What  is  the  purpose  of  the  electroscope  ? 

2.  In  an  electroscope,  why  do  the  leaves  separate  when  a  charged  body  is  brought 
near  its  knob  ? 

3.  How  can  the  character  of  a  charge  be  determined  by  means  of  an  electroscope  ? 

4.  How  can  an  electroscope  be  charged  by  induction  ? 

5.  Why  are  light  bodies  drawn  to  charged  bodies  ? 

6.  If  a  light  ball  is  drawn  to  a  charged  body  will  it  stick  to  it  ?     What  will  happen  ? 

7.  Is  there  any  attraction  between  two  heavy  bodies,  one  of  which  is  charged  ? 

8.  What  effect  has  the  size  of  two  charges  upon  their  mutual  attraction  or 
repulsion  ? 

9.  What  effect  has  the  distance  between  two  charges  upon  their  attraction  or 
repulsion  ? 

10.  If  two  small  balls,  I  inch  apart,  each  having  a  charge  cf  -2.  mi'"onths  of  a 
coulomb,  attract  with  a  known  force,  how  much  harder  will  they  pull  if  the  charge 
on  one  is  increased  to  4  millionths  of  a  coulomb?     If  both  charges  are  increased  to  4 
millionths  of  a  coulomb  ? 

11.  If  the  two  ball?  of  Question  10  are  2  inches  apart,  how  much  will  their  attrac- 
tion be  decreased  ?     If  4  inches  ? 

1 2.  If  two  charged  balls  have  their  charges  doubled,  and  are  at  the  same  time 
separated  to   twice  the  distance  they  were  at  first,  will  their  force   of  attraction 
or  repulsion  be  changed  ?     Why  not  ? 

13.  Would  the  attraction  between  two  charged  balls  be  the  same  through  oil  as  it 
is  through  air  ? 

14.  What  is  a  coulomb  ? 

15.  Why  do  two  charges  attract  or  repel  each  other  in  the   proportion  of  the 
product  of  their  charges  ? 

1 6.  What  is  an  electrometer  ? 

17.  On  what  part  of  a  conductor  does  electricity  tend  to  stay  ? 

18.  If  a  deep  pan  is  charged,  where  is  the  charge  to  be  found  ? 

19.  Why  will  a  charge  tend  to  stay  on  the  outside  of  a  hollow  cylinder  ? 

20.  How  can  an  object  be  protected  from  electric  induction  ? 

21.  What  is  an  electric  screen  ? 


CHAPTER    III 

ELECTRICAL    POTENTIAL,    El  .    AND 

ELECTRICAL    CAPACITY 

16.   Difference  of  Level  or  Potential.  —  Many  of  the   simpler 

ua  of  electricity  may  be  illustrated  by  the  action  or" 

•  efully  bear  in  mind  that  the  comparison  ly  for 

ii<  nee,  and  that  electricity  is  not  a  fluid,  but  i  form  of 

or  the  effect  of  a  vibration,  in  the  ether.1    It  must  ah.-.,-  be  remem- 

,  in  using  these  comparisons,  that  we  do  not  touch  upon  the  true 

•  ot  electricity,  which  is  not  known,  but  only  upon  the  laws  of  its 

experimentally  determined.     Also,  that  while 

gas  may  be  directly  perceived  by  our  senses,  electricity  is 

impalpable,  —  that  is,  ii  be  direct!  y  our 

'He  only  way  in  which  we  may  recognize  it  is  by  its 

various  effects. 

If  we  consider  two  vessels  oif  water  ^s  connected  by  a 

hose,  it  is  evident  at  once  from  our  ordinary  e.v"'rience  that  water  will 
flow  from  the  \  r  level  to  the  vessel  of  lowCT  level.     1 

'•iere  is  a  tendency  for  the  water  to  flow  on  account  of 
in  level  which  causes  a  pressure,  or  moth  e  force,  which  is  measured 

•  Difference  of  Level  or  Potential  between  the  water  in  the  two 

If  thr  .re  placed  at  1  ,1  flow. 

nnlogy  may  be  used  to  illustrate  the  flow  of  electricity  between 

at  the  instant  that  they  are  com  -  Aether 

by  a  wire,  urn:  inditions  mentioned  on  page  6.      One  of  the 

iold  a  positiv  >iher  a  ne 

charge,  and  t  ic  flow  is  considered  to  b< 
body  to  the  negatively  cl  !y.    Considering  tlwt  the  body  with  the 

1  Article  2. 
16 


ELECTRICAL   POTENTIAL 


positive  charge  is  at  a  higher  electrical  level,  or  potential,  than  the  body 
with  the  negative  charge,  we  may  say  that  the  flow  of  electricity  is 
caused  by  the  difference  of  electrical  level  of  the  charges  on  the  bodies, 
which  results  in  an  Electrical  Pressure  or  Electromotive  Force;  No  flow 
of  electricity  would  occur  if  the  bodies  were  at  equal  electrical  levels  at 
the  time  the  wire  connected  them. 

17.  Relative  Potentials. — The  terms  "  positive  charge  "  and  "  negative 
charge  "  are  by  this  view  shown  to  be  terms  to  indicate  that  one  body  is 
at  a  higher  potential  than  the  other,  that  is,  that  electricity  will  flow  from 
one  to  the  other  if  they  are  put  in  contact ;  and  we  may  therefore  have 


n 


FIG.  5.  — Illustration  of  Relative  Potentials. 

several  charged  bodies,  one  of  which  is  negative  to  some  and  positive  to 
others,  just  as  amongst  several  vessels  of  water,  one  may  be  lower  in 
level  than  some  and  higher  in  level  than  others. 

The  earth  is  usually  considered  to  be  at  zero  electric  potential  or  level, 
though  its  potential  varies  with  climatic  and  other  conditions ;  exactly 
as  the  average  height  of  the  seas  may  be  considered  the  zero  level  from 
which  to  measure  the  height  of  our  water  vessels. 

An  illustration  of  what  is  meant  by  the  relative  potentials  of  bodies 
may  be  had  by  reference  to  Figure  5.  We  will  assume  for  conven- 
ience that  the  horizontal  line  marked  OO  is  the  zero  level  of  the  surface 
of  the  sea.  Then  the  two  tanks  A  and  B  will  be  at  a  positive  poten- 


1 8  ELECTRICITY  AND   MAGNETISM 

tial,  because  they  are  at  higher  levels,  while  C  and  D  will  be  negative, 
because  they  are  below  the  zero  level.  The  water  in  B  may  be  consid- 
ered to  be  negative  with  reference  to  the  water  in  A,  because  it  is  of 
lower  level,  though,  as  we  are  referring  them  to  the  sea  level  as  zero,  it 
is  less  confusing  to  consider  them  both  positive,  but  A  of  greater  poten- 
tial than  B.  The  difference  of  potential  between  any  two  of  the  bodies 
of  water  will  be  directly  proportional  to  their  difference  of  levels,  irre- 
spective of  whether  they  are  positive  or  negative  with  reference  to  the 
sea  level. 

When  we  say  that  an  electric  charge  is  "  positive,"  we  usually  mean 
that  the  charge  is  of  a  potential  which  is  higher  than  that  of  our  assumed 
zero,  which  is  the  earth ;  and  when  we  say  that  an  electric  charge  is 
"negative,"  we  also  usually  mean  that  the  charge  is  of  a  potential  which 
is  lower  than  that  of  our  assumed  zero.  Such  positive  and  negative 
charges  are  always  on  opposite  sides,  respectively,  of  the  potential  of  the 
space  with  which  they  are  surrounded,  and  they  attract  each  other. 
Two  positive  charges  (and  likewise  two  negative  charges),  on  the  other 
hand,  though  they  may  differ  from  each  other  in  relative  potential, 
always  repel  each  other  because  they  are  on  the  same  side  of  the  poten- 
tial of  the  space  with  which  they  are  surrounded. 

All  the  space  immediately  around  the  earth  partakes  of  its  potential, 
except  where  the  potential  is  disturbed  by  the  influence  of  local  charges. 
This  space,  which  is  affected  with  the  earth's  potential,  is  called  the  Elec- 
tric Field  of  the  earth,  or  the  earth's  Electric  Field  of  Force. 

18.  Friction  Machines  for  generating  Electricity.  —  From  the  pre- 
ceding articles  it  is  evident  that  a  simple  machine  may  be  made  for 
the  generation  of  electricity  by  an  arrangement  for  continuously  rub- 
bing glass  with  silk  or  other  similar  material,  with  some  device  added 
for  collecting  the  electricity  which  is  developed.  A  German  named  Von 
Guericke  first  built  such  a  machine  in  1 650.  In  this  a  large  ball  of  sulphur 
was  revolved.  When  any  person  pressed  his  dry  hands  upon  the  sulphur 
ball  the  friction  generated  electricity,  and  his  body  became  charged. 
Later,  a  glass  cylinder  or  plate  and  a  rubber  of  silk  or  leather  came 
into  use. 

In  such  machines,  the  charge  upon  the  glass  is  usually  collected  by 
induction.  A  row  of  points,  called  a  Comb,  attached  to  an  insulated 


ELECTRICAL   MACHINES 


GLASS  CYLINDER 


FIG.  6.  —  Glass  Cylinder  Electrical  Machine. 


brass  block,  is  presented  to  the  charged  surface  of  the  glass  (Fig.  6). 
The  positive  charge  on  the  glass  causes  the  far  side  of  the  brass  con- 
ductor to  become  positively  charged,  and  the  row  of  points  to  become 
negatively  charged.  The 
particles  of  air  surround- 
ing and  in  contact  with 
the  points  become  nega- 
tively charged,  and  are 
repelled  off  to  the  posi- 
tively charged  glass.  This 
leaves  the  brass  conduc- 
tor with  a  positive  charge, 

and  the  negative  charge  of  the  air  particles  neutralizes  the  positive 
charge  of  the  glass,  which  is  therefore  ready  to  be  again  excited  as  it 
again  moves  around  to  the  rubber.  The  action  is  continuous  while  the 
glass  is  revolved. 

By  sprinkling  the  rubber  with  a  conducting  powder  or  an  amalgam 
made  with  mercury,  the  negative  charge  of  the  rubber  may  also  be  drawn 
away.  If  the  positively  charged  brass  conductor  is  then  connected  by  a 
wire  to  the  rubber,  a  continuous  flow  of  electricity  will  pass  from  the  brass 
conductor  to  the  rubber.  If  there  is  a  small  break  in  the  wire  the  elec- 
tricity will  jump  across  it  in  the  form  of  a  spark. 

The  friction  of  a  jet  of  wet  steam  passing  through  a  wooden  nozzle, 
and  many  other  methods,  may  be  used  to  generate  electricity  in  a  simi- 
lar way  by  friction. 

19.  Induction  Machines;  the  Electrophorus.  —  The  quantity  of  elec- 
tricity generated  in  a  reasonable  time  by  frictional  methods  is  compara- 
tively small,  and  machines  operating  by  induction  are  more  used.  The 
simplest  device  of  this  kind  is  called  an  Elec- 
trophorus. This  consists  of  a  plate  of  sulphur, 
vulcanized  rubber,  or  similar  material,  and  a 
metal  plate  or  cover  with  an  insulating  handle 
(Fig.  7).  Rubbing  the  sulphur  or  rubber  with 
flannel  electrifies  it  negatively.  When  the  cover 
is  set  down  it  touches  the  base  at  only  a  few 
FIG.  7.  —  Electrophorus.  points  on  account  of  its  roughness,  and  it  be- 


INSULATINQ 
HANDLE 


2O 


ELECTRICITY  AND    MAGNETISM 


FIG.  8.  —  Illustration  of 
the  Induced  Charges 
on  the  Cover  of  an 
Electrophorus. 


comes  electrified  by  induction  in  the  manner  illustrated  in  Figure  8. 
The  negative  induced  charge  may  be  allowed  to 
escape  into  the  operator's  body  by  touching  the 
cover  with  a  finger,  as  explained  in  Article  8.  The 
cover  then  remains  with  a  positive  charge,  which 
may  be  used  to  charge  other  bodies. 

The  process  of  charging  the  cover  may  be 
repeated  again  and  again  without  affecting  the 
charge  on  the  base,  but  the  latter  will  be  slowly 
dissipated  through  dampness  in  the  air. 
20.  The  Holtz  Induction  Machine.  —  What  is  known  as  a  Holtz  elec- 
tric machine  may  be  roughly  described  as  an  automatic  electrophorus. 
This  consists  of  two  parallel  plates  of  glass,  one  of  which  is  mounted  to 
rotate,  with  certain  inducting  and  collecting  devices  (Pig.  9).  The  fol- 
lowing is  a  brief  explanation  of  the  action  of  this  machine  :  At  opposite 
points  on  the  stationary  plate  holes  or  windows  are  cut,  and  over  these 
are  pasted  pieces  of  paper  called 
Sectors.  These  are  given  opposite 
charges  by  means  of  rubbed  rods 
of  glass  and  sealing-wax,  or  by 
other  means.  In  front  of  the  re- 
volving plate  opposite  each  sector 
is  a  comb.  The  charges  on  the 
sectors  act  indirectly  on  the  combs 
and  the  conductors  attached  to 
them,  so  that  the  knobs  that  ter- 
minate the  conductors  are  charged 
with  opposite  kinds  of  electricity. 
The  electricity  which  is  attracted 
into  the  combs  flows  off  on  to  the 
revolving  plate  exactly  as  was  ex- 
plained in  the  case  of  the  cylinder 
friction  machine,  and  charges  it  as 
shown  in  Figure  10.  The  charges 
on  the  revolving  glass  are  carried  around  under  the  opposite  combs, 
and  act  inductively  on  them,  and  are  then  neutralized  by  the  charges 


REVOLVING  PLATE 


STATIONARY  PLATE 


FIG.  9.  —  Holtz  Machine. 


ELECTRICAL   MACHINES 


21 


REVOLVING  PUAJE 


•f     -KNOBS 


FiG.  10.  —  Illustration  of  the  Operation 
of  a  Holtz  Machine. 


on  the  streams  of  air  particles  passing  off  the  combs.  If  the  two  knobs 
are  placed  in  connection,  a  flow  of  electricity  passes  through  the  con- 
ductors out  of  the  combs  on  to  the 
revolving  plate,  which  is  thus  kept 
charged,  as  in  Figure  10,  and  the 
current  of  electricity  continues  as 
long  as  the  plate  is  revolved.  If 
the  plate  is  revolved  with  sufficient 
rapidity  a  spark  will  jump  from  knob 
to  knob,  thus  completing  the  circuit, 
even  when  the  knobs  are  a  consider- 
able distance  apart. 

In  starting  the  machine,  it  is  really 
sufficient  to  charge  only  one  of  the 

sectors,  as  the  other  will  then  become  charged  through  the  action  of 
the  machine.  It  is  not  necessary  to  go  fully  into  the  action  of  these 
machines,  or  into  that  of  various  devices  used  to  increase  their  effective- 
ness and  make  them  self-exciting. 

21.  Hydraulic  Analogy.  —  The  action  of  Holtz  and  similar  machines 
may  be  compared,  in  a  rough  but  handy  analogy,  to  pumps  for  circulat- 
ing water  or  gas  through  a  system  of  pipes.  The  machines  maybe 
considered  to  be  machines  for  pumping  electricity. 

Suppose  two  deep  tubs  to  be  placed  side  by  side,  and  a  pump  to  be 
connected  in  a  pipe  leading  from  the  bottom  of  one  to  the  bottom  of 
the  other.  If  the  tubs  are  partly  full  of  water  and  the  pump  is  started, 
water  will  be  drawn  from  one  tank  and  sent  into  the  other  one,  thus 
raising  the  water  level  in  the  latter.  If  an  overflow  pipe  is  carried  from 
the  upper  tub  to  the  lower  one,  the  overflow  will  run  back  into  the 
lower  tank,  and  the  water  will  be  simply  circulated  by  the  pump  through 
the  system  of  pipes  between  the  two  tanks. 

This  is  similar  to  the  conditions  of  an  electrical  machine  when  the 
positive  and  negative  terminals  are  connected  together  or  sparks  are 
passing  between  them. 

Now,  if  the  overflow  pipe  is  stopped  up,  and  drip  pans  are  arranged 
so  that  water  from  the  upper  tank  cannot  run  down  into  the  lower  one, 
the  pump  will  soon  empty  the  lower  tank,  after  which  it  may  continue 


22  ELECTRICITY   AND    MAGNETISM 

to  run,  but  it  cannot  pump  any  water,  and  no  stream  will  flow  through 
the  pipes. 

In  the  same  way,  if  the  two  conductors  of  an  electric  machine  are 
not  connected,  and  are  too  far  apart  for  a  spark  to  pass  between  them, 
the  conductors  will  be  strongly  charged  with  opposite  kinds  of  elec- 
tricity (that  is,  their  difference  of  electrical  level  or  potential  becomes 
great),  but  then  the  action  of  the  machine  in  circulating  electricity  must 
cease  until  a  path  is  provided  for  the  current  to  flow. 

22.  The  Ampere. — The  quantity  of  water  circulated  by  the  pump 
depends  upon  the  pressure  which  it  produces,  and  upon  the  size  of  the 
connecting  pipes ;  and  a  similar  rule  holds  for  the  circulation  of  elec- 
tricity by  an  electrical  machine.     The  volume  of  the  stream  of  water 
may  be  designated  as  a  certain  number  of  gallons  or  cubic  feet  per 
second.     In  the  same  way  the  volume  of  a  current  of  electricity  may 
be  designated  as  one  which  conveys  a  certain  number  of  coulombs  per 
second.     An  electric  current  carrying  one  coulomb  per  second  is  called 
a  current  of  one  Ampere,  and  the  volume  of  an  electric  current  is  always 
given  in  amperes.     This  name  was  given  in  honor  of  a  great  French 
scientist  whose  name  was  Ampere. 

23.  The  Volt.  —  To  pass  a  stream  of  a  certain  number  of  gallons  per 
minute  through  a  certain  pipe  demands  the  application  of  a  certain 
pressure  to  overcome  the   frictional   resistance.      In  the  same  way  it 
requires  a  certain  Electrical   Pressure,   or  difference    of  potential,   to 
pass  a  given  electrical  current  through  any  conducting  wire,  on  account 
of  the  resistance  which  the  wire  offers  to  the  flow  of  the  electricity. 
The  resistance  to  the  passage  of  electricity,  or  the  Electrical  Resistance, 
in  any  material,  is  the  reciprocal  or  opposite  of  its  conducting  power. 
The  greater  its  conducting  power,  the  less  is  its  electrical  resistance. 

We  usually  measure  water  pressure,  or  the  pressure  of  gas,  in  pounds 
per  square  inch,  or  in  feet  difference  of  level,  or  head.  The  correspond- 
ing unit  of  electrical  pressure  is  a  Volt,  which  was  named  after  Volta,  a 
great  Italian  scientist. 

Returning  again  to  the  pump  and  tanks,  —  when  the  pump  is  set  in 
motion,  it  sets  up  a  difference  of  pressure  which  may  be  measured  by  a 
gauge,  and  this  starts  the  water  to  flowing  if  it  has  an  outlet.  In  the 
same  way  we  may  look  upon  electrical  machines  as  setting  up  a  differ- 


ELECTRICAL  MACHINES  23 

ence  of  electrical  pressure  (which  may  be  measured  by  a  proper  electri- 
cal instrument),  and  this  starts  the  electricity  to  flowing  if  it  has  an 
outlet. 

This  leads  us  to  the  necessity  of  considering  a  positive  charge  of  elec- 
tricity as  electricity  at  high  electrical  pressure  or  high  potential,  and  a 
negative  charge  as  electricity  at  low  electrical  pressure  or  low  potential.1 

When  a  point  of  high  electrical  pressure  is  connected  by  a  conducting 
wire  to  a  point  of  lower  pressure,  electricity  will  flow  from  the  higher 
point  to  the  lower,  until  the  pressure  is  equalized,  unless  the  difference 
of  pressure  is  continually  kept  up  by  a  machine ;  exactly  as  has  been 
explained,  where  two  tanks,  standing  side  by  side,  are  filled  with  water 
to  different  heights,  and  are  connected  by  a  pipe,  —  in  which  case  water 
will  flow  from  one  to  the  other  until  the  level  is  the  same  in  both.2 

24.  Pressure  produced  by  Electrical  Machines.  —  Before  leaving  the 
question  of  electrical  machines  working  by  friction  and  induction,  it  is 
well  to  call  attention  to  the  great  pressure  of  the  electricity  generated 
by  them.     This  is  shown  by  the  sparks  which  may  be  caused  by  them 
to  pass  through  the  air,  or  even  to  pierce  wood,  glass,  or  other  solid 
insulators.     These  effects  may  be  called  miniature  lightning  effects,  for 
lightning  is  simply  caused  by  the  passage  through  the  air  of  a  current 
of  electricity  under  enormous  pressure.     Thunder  is  like  the  crackle  of 
the  spark  from  an  electrical  machine  greatly  magnified. 

While  the  electrical  pressure  generated  by  these  machines  is  very 
great,  the  quantity  of  electricity  generated  is  quite  small,  and  for  com- 
mercial purposes,  in  which  a  considerable  volume  of  electricity  is 
needed,  other  methods  of  generating  the  current  are  used.  These  are 
described  in  later  chapters. 

25.  Lightning.  —  The  identity  of  electrical  discharges  and  lightning 
was  demonstrated  at  the  instance  of  Benjamin  Franklin   (one   of  the 
boldest  experimenters  and  most  brilliant  thinkers  the  world  has  ever 
produced).     He  himself  secured  unimpeachable  evidence  of  this  iden- 
tity in  1752,  by  his  classical  experiment  of  drawing  an  electrical  dis- 
charge from  a  thunder  cloud  along  the  string  of  a  kite.     It  was  through 
Franklin's  suggestion  that  lightning  rods  came  to  be  erected,  and  by 
1782  at  least  four  hundred  were  in  use  in  Philadelphia.     It  has  never 

1  Articles  16  and  17.  2  Article  16. 


24  ELECTRICITY   AND    MAGNETISM 

yet  been  conclusively  determined  how  the  clouds  get  their  great  charges 
of  electricity  which  produce  the  lightning  strokes. 

26.  Electrical  Capacity.  —  If  the  bottom  of  one  of  the  tanks  described 
in  the  first  articles  of  the  chapter,  which  is  filled  with  water,  is  con- 
nected by  means  of  a  pipe  to  the  bottom  of  its  mate  of  different  diameter, 
which  stands  on  the  same  level,  the  water  will  flow  into  the  second  tank 
until  it  stands  at  the  same  height  in  both.     The  quantity  of  water  in 
each  vessel,  when  the  flow  has  ceased,  is  proportional  to  the  capacity  of 
the  vessel.     During  the  flow,  the  water  falls  in  one  vessel  and  rises  in  the 
other.     In  the  same  way  if  a  conductor,  such  as  a  brass  ball  carrying  an 
electric  charge,  is  touched  by  an  uncharged  conductor,  part  of  the  charge 
flows  to  the  second  conductor.     During  the  flow  the  electric  pressure 
of  one  conductor  falls  and  the  pressure  of  the  other  rises.     After  the 
flow  has  ceased,  the  electrical  pressures  of  the  two  conductors  are  equal. 
The  quantities  of  electricity  on  the  two  conductors  are  not  equal  unless 
the  conductors  are  exactly  similar,  but  the  quantity  on  each  will  depend 
upon  its  capacity  to  hold  electricity,  or  its  Electrical  Capacity.     The 
electrical  capacity  of  a  conductor   depends  upon  its  size,  shape,  and 
surroundings.     It  is  measured  by  the  number  of  coulombs  of  electricity 
required  to  raise  the  electrical  pressure  of  the  conductor  one  volt,  exactly 
as  the  capacity  of  a  cylindrical  tank  is  measured  by  the  number  of  gal- 
lons of  water  required  to  fill  it  to  the  depth,  or  head,  of  one  foot. 

The  electrical  pressure  of  a  conductor  carrying  a  charge  of  electricity 
is  ordinarily  reckoned  as  the  difference  between  it  and  the  average  elec- 
trical pressure  of  the  earth's  surface,  which  is  called  zero.  This  is  simi- 
lar to  the  reference  of  levels  or  heights  to  the  sea  level  as  a  zero  point 
from  which  to  start. 

27.  The  Farad.  —  When  the  pressure  of  a  conductor  is  raised  one  volt 
by  the  charge  of  one  coulomb,  the  conductor  is  said  to  have  a  capacity 
of  one  Farad,  after  Faraday,  the  distinguished  English  scientist. 

28.  Condensers.  — The  presence  of  charges  of  an  opposite  sign  near 
a  charged  conductor  has  a  remarkable  influence  on  the  conductor's 
capacity.     For  instance,  if  pieces  of  tinfoil  are  pasted  upon  the  two 
sides  of  a  sheet  of  mica,  and  the  two  tinfoil  coatings  are  given  opposite 
charges,  the  charges  act  inductively  on  each  other  and  increase  the 
capacities  of  the  coatings  by  modifying  their  relative  potentials.     Such 


CONDENSERS  25 

an  arrangement  is  called  a  Condenser.  The  tinfoil  sheets  are  called  the 
Coatings  or  Plates  of  the  condenser,  and  the  insulating  material  is  called 
the  Dielectric.  The  coatings  of  a  condenser  may  be  made  of  any  con- 
ducting material,  and  the  dielectric  of  any  insulating  material. 

The  combined  capacity  of  the  coatings  is  the  capacity  of  the  con- 
denser. A  condenser  has  a  capacity  of  one  farad  when  the  transfer  of 
one  coulomb  of  electricity  from  one  plate  to  the  other  changes  the  difference 
of  electrical  pressure,  or  potential,  between  the  plates  by  one  volt;  and  the 
quantity  of  electricity  in  a  charged  condenser  is  equal  to  the  product  of  the 
capacity  of  the  condenser  (in  farads}  with  the  difference  of  pressure  be- 
tween the  plates  (in  volts) .  This  may  be  represented  algebraically  by 
the  expressions 

Q  =  SV,   and  hence   S=  Q/V  and    V=  Q/S, 

if  Q  is  taken  to  represent  coulombs,  6"  to  represent  farads,  and  V  to 
represent  volts.  To  "  charge  a  condenser  with  a  certain  quantity  of  elec- 
tricity "  means  that  a  positive  charge  of  the  given  .quantity  is  placed  upon 
one  plate  and  an  equal  negative  charge  on  the  other  plate. 

A  farad  is  a  much  larger  capacity  than  is  ordiparijy  found  in  electrical 
work,  so  that  capacity  is  usually  reckoned  in  millionths  of  a  farad,  or 
Microfarads  (from  micro,  meaning  little). 

Example.  —  What  pressure  is  required  to  charge  a  condenser  of 
10  microfarads  capacity  with  .03  coulombs  of  electricity.  Ans.  3000  volts. 

A  condenser  may  be  charged  in  either  of  two  ways  :  — 

First,  by  connecting  one  plate  to  earth  and  placing  the  charge  on 
the  other  plate,  when  the  required  opposite  charge  will  collect  on  the 
grounded  plate  by  induction. 

Second,  by  connecting  the  two  plates  of  the  condenser  to  the  two  ter- 
minals of  an  electrical  machine,  or  other  source  of  electricity,  when  the 
charge  is  communicated  by  the  machine,  which  acts  as  an  electricity  pump. 

Every  electrical  conductor,  as  we  have  seen,  has  capacity,  and  when 
an  insulated  wire  is  laid  in  the  earth  or  is  strung  overhead  it  becomes 
one  plate  of  a  condenser.  The  other  plate  of  the  condenser  is  the  earth, 
and  the  dielectric  is  the  insulating  covering  of  the  wire,  or  the  air  which 
is  between  it  and  the  earth.  The  capacity  of  a  wire  has  a  great  deal 
of  effect  on  its  usefulness  in  telephone  service.  Every  hundredth  of  a 


26 


ELECTRICITY   AND   MAGNETISM 


microfarad  per  mile  of  conductor  reduces  very  considerably  the  distance 
through  which  the  telephone  will  work  satisfactorily.  The  capacity  of 
ocean  cables  is  also  a  matter  of  much  importance,  and  capacity  effects 
are  of  importance  in  telegraphy  and  in  the  transmission  of  power  by 
alternating  currents  of  electricity. 

29.  The  Leyden  Jar.  —  For  experimental  purposes  a  condenser  is 
sometimes  made  by  coating  the  inside  and  outside  of  a  glass  jar  with 
tinfoil.  Such  an  arrangement  is  called  a  Leyden  Jar.  In  this  case  the 


FIG.  ii. —  Battery  of  Leyden  Jars. 

inside  and  outside  coatings,  respectively,  are  the  condenser  plates,  and 
the  intervening  glass  of  the  jar  the  dielectric.  A  group  of  nine  Leyden 
jars  in  a  box  is  illustrated  in  Figure  1 1 . 


QUESTIONS 

1.  Explain  the  meaning  of  the  phrase  difference  of  potential. 

2.  Compare  difference  of  electric  potential  with  difference  of  water  levels. 

3.  What  is  the  assumed  zero  of  electrical  potential  ? 

4.  Can  a  charge  be  negative  with  reference  to  another  and  still  be  at  a  higher 
potential  than  the  earth  ? 

5.  What  potentials  are  usually  considered  to  be  positive  and  what  negative? 
What  is  used  as  a  standard  to  determine  this? 

6.  What  is  a  friction  electrical  machine? 

7.  How  can  you  make  a  friction  machine  for  generating  electricity? 


CONDENSERS  2  / 

8.  What  is  an  induction  electrical  machine? 

9.  What  is  an  electrophorus?  , 

10.  How  is  an  electrophorus  used  ? 

11.  When  an  electrophorus  cover  is  set  upon  the  charged  plate,  why  does  it  not 
rob  the  plate  of  its  charge  ? 

12.  If  the  plate  of  an  electrophorus  is  charged  positively  and  the  cover  placed 
upon  it,  why  will  the  cover  retain  a  negative  charge  if  it  is  touched  by  the  finger 
before  it  is  removed  ? 

13.  Describe  a  Holtz  machine. 

14.  What  are  the  sectors  in  a  Holtz  machine  for? 

15.  Explain  the  complete  action  in  a  Holtz  machine. 

1 6.  Compare  the  action  of  electrical  machines  with  a  pump. 

1 7.  What  is  an  ampere  ? 

1 8.  If  10  coulombs  of  electricity  flow  through  a  conductor  in  a  second,  how  many 
amperes  compose  the  current  ? 

19.  If  50  coulombs  of  electricity  flow  through  a  conductor  in  10  seconds,  how 
strong  is  the  current  ? 

20.  If  a  current  of  10  amperes  is  carried  by  a  conductor  for  10  seconds,  how  many 
coulombs  of  electricity  pass  through  the  conductor? 

21.  What  is  a  volt  ? 

22.  What  is  it  that  is  measured  or  expressed  in  volts? 

23.  What  is  meant  by  the  resistance  of  a  conductor? 

24.  What  relation  has  resistance  to  conductivity? 

25.  Why  are  friction  or  induction  electrical  machines  of  small  commercial  value  ? 

26.  What  is  electrical  capacity? 

27.  Compare  the  capacity  of  a  conductor  for  holding  electricity  with  that  of  a 
cylindrical  tank  for  holding  water. 

28.  What  is  a  farad  ? 

29.  After  whom  was  this  unit  of  capacity  named  ? 

30.  What  is  a  condenser? 

31.  What  is  a  dielectric? 

32.  Why  is  the  combined  capacity  of  two  plates  increased  when  they  are  brought 
close  together? 

33.  How  may  a  condenser  be  charged  ? 

34.  Do  all  conductors  have  capacity? 

35.  When  has  a  condenser  a  capacity  of  I  farad  ? 

36.  What  capacity  has  a  condenser  if  a  charge  of  20  coulombs  raises  the  pressure 
between  its  terminals  by  one  volt  ? 

37.  Would    the  condenser  of  the   preceding   question   be  very  large   or  small 
compared  with  those  likely  to  be  met  with  in  electrical  work  ? 

38.  Describe  a  Leyden  jar. 


CHAPTER   IV 


ELECTRIC  BATTERIES,  OR  APPLIANCES  FOR  TRANSFORMING  CHEM- 
ICAL  ENERGY   INTO   ELECTRICAL  ENERGY 

30.  Electric  Batteries.  —  One  of  the  effects  of  chemical  action  is  to 
give  out  heat.     When  wood  or  coal  is  burned,  the  carbon  of  the  burning 
material  combines  with  oxygen  from  the  air,  and  heat  is  given  out  as  the 
result  of  the  chemical  combination  which  we  call  Combustion  or  burning. 
In  the  same  way,  if  zinc  is  dissolved  in  sulphuric  acid,  the  acid  combines 
with  the  zinc,  and  heat  is  given  off  as  the  result  of  the  chemical  combina- 
tion.    This  heat  represents  a  certain  energy  or  capacity  for  doing  work. 
It  has  been  found  that  under  certain  conditions  the  energy  thus  repre- 
sented by  chemical  action  may  be  converted  into  an  electric  current,  and, 
taking  advantage  of  this,  we  get  Electric  Batteries. 

Electric  currents  produced  by  chemical  action  were  first  observed  and 
studied  about  the  end  of  the  last  century  by  Galvani  and  Volta,  both  of  whom 
were  Italian  scientists.  Volta  will  be  recognized  as  the  man  from  whose 
name  comes  the  word  "  volt,"  the  name  of  the  unit  of  electrical  pressure.1 

31.  Two  Different  Metals  dipped  in  a  Liquid. — When  two  plates  of 

different  metals  are  placed  in  a  liquid 


ZINC  PLATE 


+  COPPER  PLATE 


FIG.  12.  —  Simple  Battery  Cell. 


they  ^o  not  touch  each  other, 
as  is  shown  in  Figure  12,  and  the 
liquid  is  one  which  is  inclined  to 
attack  them  chemically,  one  of  the 
plates  becomes  positively  charged, 
and  the  other  negatively  charged, 
with  electricity.  The  charges  are 
so  minute  that  they  cannot  be  dis- 
tinguished by  the  electroscope  pre- 


viously explained,  but  a  delicate  electrometer  will  distinguish  and  meas- 
ure them. 

i  Article  23. 
28 


ELECTRIC   BATTERIES 


If  the  metal  plates  are  connected  by  a  wire,  a  current  flows  through  it 
from  one  plate  to  the  other,  and  this  may  be  readily  distinguished  by  its 
effects,  which  are  described  in  later  chapters.  The  positive  or  high 
pressure  plate  is  the  one  which  is  attacked  least  readily  by  the  liquid. 

32.  Voltaic  Cell.  —  It  is  almost  exactly  a  century  since  Volta  discov- 
ered that  an  electromotive  force  may  be  set  up  through  actions,  as  above 
explained,  in  a  more  convenient  manner  than  by  charging  two  bodies 
with  unlike  charges.  His  discovery  is  at  the  foundation  of  a  great  deal 
of  our  electrical  work  of  to-day,  and  in  simple  words  may  be  explained 
as  follows  :  When  a  complete  conducting  path  or  ring  is  made  up  of 
metal  strips  or  wires,  there  is  no  electric  action  (provided  all  parts  of 
the  path  are  at  the  same  temperature,  and  in  like  conditions  in  other  re- 
spects), no  matter  how  many  different  pieces  of  metal  of  various  kinds 
are  introduced  into  the  ring. 

Now,  if  an  opening  is  made  in  the  metal  ring  where  two  different 
metals  join,  and  the  two  ends  are  dipped  into  a  little  acid,  or  a  solution 
of  salt,  or  some  other  chemically  active  liquid,  a  difference  of  potential 
is  at  once  set  up  and  an  electric  current  flows  around  the  ring.  Partic- 
ular attention  should  be  given  to  the  fact  that  the  two  ends  of  the  ring 
dipped  into  the  fluid  must  be  composed  of  different  metals  in  order  that 
a  current  may  flow.  It  therefore  requires  that 
at  least  three  different  conductors  (in  our  illus- 
tration, two  different  metals  and  a  liquid)  shall 
be  joined  in  a  ring  if  a  current  flow  is  to  be 
obtained ;  and  at  least  one  of  the  conductors, 
as  for  instance  the  liquid,  must  be  of  a  differ- 
ent chemical  class  from  the  others. 

The  arrangement  under  consideration  is 
illustrated  in  Figure  13,  which  shows  a  ring 
composed  of  joined  wires  of  zinc  and  copper, 
with  their  free  ends  dipping  into  a  tumbler 
holding  dilute  sulphuric  acid.  The  arrows 
show  the  direction  of  the  flow  of  current. 
If  we  sever  the  wires  outside  of  the  liquid 
the  electromotive  force  will  still  remain,  but  no  current  can  then  flow 
because  the  conducting  path  is  broken.  Upon  again  joining  the  wires, 


ARROWS  SHOW  DIRECTION 
OF  FLOW  OF  CURRENT 

FIG.  13. —  Illustration  of  Sim- 
ple Voltaic  Cell. 


ELECTRICITY   AND    MAGNETISM 


JCu 


Etc. 


the  current  again  can  and  does  flow.     Such  an  arrangement  is  generally 
called  a  Voltaic  Cell,  after  the  name  of  Volta. 

In  order  to  utilize  the  difference  of  potential,  which  we  have  been 
discussing,  Volta  first  constructed  what  has  since  been  called  Volta's 
Pile.  It  consists  of  a  pile  of  disks  of  zinc,  cloth,  and  copper  stacked 

up  on  each  other  in  the  order  given 
until  a  stack  as  high  as  may  be  desired 
is  obtained.  The  cloth  is  moistened  with 
acid  or  a  salt  solution.  Figure  14  illus- 
trates the  pile.  If  the  upper  copper  is 
connected  with  the  lower  zinc  plate  by 
means  of  a  wire,  a  current  flows  through 
the  connecting  wire.  When  there  are 
enough  sets  of  plates,  the  pressure  and 
current  set  up  will  be  sufficient  to  cause 
quite  a  spark,  if  the  connecting  wire  is 
suddenly  broken.  Each  of  the  sets  of 
copper  and  zinc  disks  with  acid  or  salt 
moistened  cloth  between  composes  a 
voltaic  cell,  and  their  differences  of  po- 
tentials all  added  together  give  quite  a 
pressure.  In  this  case,  as  before,  the 

combination  of  materials,  copper,  zinc,  and  moisture,  combine  to  give 
the  pressure,  while  chemical  action  furnishes  the  necessary  energy. 

Volta  also  found  that  he  could  produce  the  electrical  action  by  placing 
the  zinc  and  copper  plates  in  vessels  containing  dilute  acid,  the  liquid 
taking  the  place  of  the  moistened  cloth,  as  in  Figures  12  and  13.  This 
arrangement  forms  the  basis  of  our  electric  batteries  of  to-day. 

33.  Unit  of  Electromotive  Force  or  Pressure.  —  When  speaking  of 
the  tendency  of  electricity  to  move  from  one  body  to  another,  it  is  satis- 
factory to  use  the  term  "electrical  pressure"  instead  of  the  longer  term 
"electromotive  force."  In  dealing  with  the  flow  of  electricity,  some 
unit  for  measuring  this  pressure  must  be  adopted,  just  as  in  meas- 
uring differences  of  level  we  use  the  foot  length  as  a  unit  and  in 
measuring  the  tendency  of  water  to  flow  we  use  the  pressure  of  a  pound 
per  square  inch  as  the  unit.  The  unit  of  electrical  pressure  is  called  a 


FIG.  14.  — Volta's  Pile. 

Zn  and  Cu  represent  the  zinc  and 
copper  plates,  and  the  interven- 
ing material  is  the  moistened 
cloth. 


ELECTRIC   BATTERIES  31 

volt,1  after  the  name  of  Volta,  and  one  volt  of  electrical  pressure  is  a 
little  more  than  the  electromotive  force  of  a  Voltaic  cell  with  zinc  and 
copper  plates  and  dilute  sulphuric  acid  for  the  liquid. 

34.  Energy  expended  in  Continuous  Flow.  —  When  two  insulated 
conductors  at  different  electrical  pressures  are  connected  by  a  wire,  a 
brief  current  flows ;  just  as  a  current  of  water  flows  through  the  pipe 
connecting  two  tanks  in  which  water  stands  at  different  levels ;  but  the 
current  ceases  as  soon  as  the  pressure  is  equalized.  In  order  that  a 
contimtous  current  may  be  produced,  a  difference  of  electrical  pressure 
must  be  continuously  supplied  in  a  closed  circuit.  If  some  method  is 
introduced  for  maintaining  the  difference  of  potential  or  pressure  (as 
would  be  the  case,  in  our  analogy  of  vessels  of  water  connected  by  a  pipe  or 
hose,  if  we  kept  pumping  the  water  from  the  lower  to  the  higher  vessel), 
the  flow  of  electricity  will  be  constant.  One  method  of  accomplishing 
this  is  by  means  of  the  electric  batteries  which  have  grown  out  of  Volta's 
discovery,  and  which  may  be  considered  for  convenience  as  chemical 
engines  for  pumping  electricity  from  a  lower  potential  to  a  higher. 

If  we  desire  to  have  a  continuous  flow  of  water  from  one  vessel  to 
another  one  at  lower  level,  it  is  necessary  to  keep  refilling  the  higher 
vessel  as  water  runs  out  of  it.  To  do  this,  the  water  must  be  raised 
from  the  lower  level  to  the  higher,  which  means  that  work  must  be  per- 
formed and  energy  expended.  There  are  various  ways  in  which  this 
energy  may  be  supplied  :  we  may  carry  the  water  up  in  hand  buckets, 
thus  using  up  animal  energy ;  we  may  divert  rain  water  or  a  flowing 
stream  into  the  higher  vessel,  thus  utilizing  the  effects  of  the  energy  from 
the  heat  of  the  sun,  by  means  of  which  the  water  was  raised  from  the 
earth  to  become  clouds ;  we  may  pump  the  water  directly  from  the 
lower  vessel  to  the  upper  one  by  means  of  a  pump  driven  by  a  steam 
engine,  thus  utilizing  the  energy  of  the  steam,  which  was  given  to  it  from 
the  heat  of  the  burning  coal. 

We  may  then  say  that  it  requires  a  continuous  expenditure  of  energy 
to  keep  water  continuously  in  circulation.  Some  of  this  energy  may  be 
recovered  by  putting  water  motors  in  the  hose  or  pipe  through  which 
the  water  flows  from  the  higher  to  the  lower  vessel,  but  much  of  the 
energy  is  lost  by  friction  in  the  pipes  and  pump. 

1  Article  23. 


32  ELECTRICITY   AND   MAGNETISM 

In  the  same  way,  it  requires  a  continuous  expenditure  of  energy  to 
keep  an  electric  current  flowing,  and  this  energy  may  be  gained  from 
the  chemical  energy  resulting  from  dissolving  zinc  or  some  other  metal 
in  an  electric  battery. 

35.  The  Battery  Cell  and  Electromotive  Force.  —  A  cup  containing 
two  plates  immersed  in  a  liquid  such  as  has  been  described,  is  also 
called  an  Electric  Battery  Cell,  and  the  plates  are  called  the  Poles  or 
Electrodes  of  the  cell.     It  is  usual  to  speak  of  the  electrode  which  is  at 
the  higher  pressure,  and  from  which  current  flows  through  an  external 
wire,  as  the  Positive  Pole  or  Positive  Plate ;  the  other  electrode  is  called 
the  Negative  Pole  or  Plate.     The  liquid  is  called  the  Electrolyte.     The 
difference  of  electrical  pressure  between  the  poles  is  called  the  electro- 
motive force  of  the  cell.   The  phrase  "electromotive  force "  means  a  force 
which  tends  to  move  electricity,  that  is,  a  difference  of  electrical  pressure 
or  potential.     This  phrase  is  often  abbreviated  into  E.M.F.     We  will 
generally  speak  of  it,  however,  as  the  electrical  pressure  of  the  cell. 

An  electric  battery  is  often  called  a  Voltaic  or  Galvanic  Battery,  and 
the  electricity  produced  by  a  battery  is  often  called  Voltaic  or  Galvanic 
Electricity,  although  the  electricity  is  exactly  the  same  as  that  produced 
by  any  other  means.  These  terms  are  applied  in  the  same  way  as  the 
terms  "  Spring  Water  "  and  "  Well  Water, "  for  instance,  are  applied  to  pure 
water  which  is  drawn  from  a  spring  or  well,  though  the  water  does  not 
differ  from  pure  water  drawn  from  other  sources. 

36.  Complete  Circuit  of  Electric  Flow.  —  When  the  poles  of  a  cell  are 
connected  by  a  wire  it  is  found  that  an  electric  current  not  or\\y  flows 
from  the  positive  to  the  negative  pole  through  the  'wire,  but  it  continues 
through  the  liquid  from  the  negative  to  the  positive  pole.     If  the  current 
is  followed  from  any  point  in  its  flow,  it  will  be  found  to  return  through 
a  complete  path  to  the  same  point,  exactly  as  water  is  circulated  by  a 
pump  through  a  system  of  pipes.     A  continuous  current  of  electricity  is 
therefore  said  to  flow  in  a  Complete  Path  or  Circuit.    A  complete  circuit 
is  often  called  a  Closed  Circuit. 

The  current  inside  the  cell,  then,  is  driven  by  the  effect  of  chemical 
action  against  a  difference  of  pressure,  just  as  water  may  be  raised  by  a 
pump  against  a  difference  of  pressure.  Outside  of  the  cell,  where  there 
is  no  restraining  action  besides  that  of  the  electrical  resistance  of  the 


ELECTRIC   BATTERIES 


33 


connecting  wire,  the  current  follows  its  own  tendency  to  flow  from  the 
point  of  high  pressure  to  that  of  low  pressure. 

37.  Electrical  Pressure  of  Cells.  —  The  magnitude  and  direction  of 
the  electrical  pressure  between  the  poles  of  a  battery  cell  depend  upon  the 
materials  in  the  plates  and  the  character  of  the  liquid.     For  instance,  if 
zinc  and  copper  compose  the  plates  of  a  cell  containing  sulphuric  acid, 
the  electrical  pressure  of  the  cell  is  about  nine -tenths  of  a  volt  and  the 
copper  plate  is  the  positive  pole.     If  two  cells  are  made  with  sulphuric 
acid  as  a  liquid,  using  zinc  and  lead  for  the  plates  of  one  cell,  and  lead 
and  copper  for  the  plates  of  the  other,  the  lead  is  the  positive  plate  in 
the  former   and  the  negative  plate  in  the  latter.     Also,  the  electrical 
pressure   developed  in  each  of  these  is  less  than  in  the  case  of  the 
zinc-copper  cell. 

In  cells  which  are  to  be  obtained  from  dealers,  the  negative  poles  or 
plates  are  nearly  always  of  zinc,  but  the  metals  composing  the  posi- 
tive plates  and  the  compositions  of  the  liquids  vary  greatly.  The  positive 
plates  are  generally  made  of  copper,  carbon,  or  platinum,  and  the  liquids 
consist  of  various  acids,  or  solutions  of  sal  ammoniac,  caustic  potash 
or  other  compounds  in  water. 

38.  Connection  in  Series.  —  If  a  number  of  cells,  such  as  the  zinc- 
copper  cell  described  above,  are  connected  in  a  series  with  the  zinc 
pole  of  one  connected  to  the 

copper  pole  of  the  next,  and 
so  on,  as  is  illustrated  in 
Figure  15,  then  the  total 
electrical  pressure  measured 
between  the  free  copper  pole 
and  the  free  zinc  pole  is  equal 
to  the  sum  of  the  pressures  de- 
veloped by  all  of  the  individual 
cells.  When  a  battery  is  con- 
nected in  this  manner  so  that  the  pressures  developed  in  the  individual 
cells  are  all  added  together,  the  cells  are  said  to  be  Connected  in  Series. 

39.  The  Pressure  of  a  Cell  is  Independent  of  its  Size.  —  The  electrical 
pressure  of  a  cell  depends  only  upon  the  nature  of  the  plates  and  the 
liquid,  and  is  entirely  independent  of  the  size  of  the  plates.     This  can  be 


FIG.  15.  — Four  Voltaic  Cells  Connected  in  Series. 

Terminals  of  copper  plates  are  marked  +. 
Terminals  of  zinc  plates  are  marked  — . 


34  ELECTRICITY   AND   MAGNETISM 

easily  proved  by  making  two  cells  out  of  tumblers  containing  dilute  sul- 
phuric acid,  in  one  of  which  are  placed  narrow  strips  of  copper  and 
zinc,  and  in  the  other  are  placed  broad  strips  of  the  metals.  If  these  are 
connected  in  series  and  the  circuit  is  closed  by  joining  the  free  poles  by 
a  wire,  a  current  flows,  as  is  shown  by  the  vigorous  chemical  action 
which  causes  bubbles  to  gather  on  the  copper  plates.  If  one  of  the  cells 
is  now  reversed,  so  that  its  copper  pole  is  connected  to  the  copper  pole 
of  the  other,  no  such  action  occurs,  which  shows  that  the  electrical 
pressures  which  now  tend  to  send  currents  in  opposite  directions  are 
equal  and  neutralize  each  other. 

40.  Polarization.  —  If  the  two  poles  of  a  zinc-copper  cell,  such  as  we 
have  been  considering,  are  connected  by  a  wire,  a  vigorous  chemical 
action  goes  on  at  first,  but  it  gradually  decreases  in  intensity  and  finally 
appears  to  stop  altogether.     This  effect  may  be  plainly  shown  by  con- 
necting an  electric  bell  in  the  circuit  of  the  cell.  When  the  circuit  is  first 
completed  the  bell  rings  loudly,  but  it  soon  weakens,  and  after  a  time 
ceases  to  ring  altogether.  If  the  cell  is  then  examined,  a  layer  of  bubbles 
may  be  found  upon  the  copper  plate.     These  bubbles  are  composed  of 
hydrogen  gas  which  is  liberated  from  the  sulphuric  acid  by  the  chemical 
action  in  the  cell.     The  effect  of  the  hydrogen  bubbles  is  twofold  :  first, 
the  hydrogen  in  contact  with  the  electrolyte  in  the  cell  tends  to  produce 
a  smaller  electric  pressure  than  is  set  up  between  the  copper  and  the  elec- 
trolyte, and  thus  the  effective  pressure  of  the  cell  is  reduced  ;  second,  the 
layer  of  bubbles  presents  a  high  resistance  to  the  flow  of  the  current. 

A  cell  which  is  made  inactive  by  a  layer  of  hydrogen  bubbles,  is  said 
to  be  Polarized,  and  the  effect  is  called  Polarization. 

41.  Depolarization.  —  In  order  that  a  cell  may  be  capable  of  working 
continuously,  some  plan  must  be  adopted  to  keep  it  from  polarizing,  or, 
as  it  is  often  called,  to  keep  it  Depolarized.     This  may  be  effected  in 
three  different   ways  :    first,  by   mechanical  action ;  second,    by  direct 
chemical  action  which  absorbs  the  hydrogen  ;    third,  by  electrochemical 
action,  by  which  the  hydrogen  is  exchanged  for  a  metal  which  is  depos- 
ited upon  the  positive  plate. 

42.  Open  and  Closed  Circuit  Cells.  —  In  all  cells  that  use  a  mechanical 
method  of  depolarization  and  in  many  that  use  a  chemical  depolarizer, 
the  depolarizing  is  effected  so  slowly  that  the  cells  cannot  give  a  long 


ELECTRIC   BATTERIES  35 

continued  steady  flow  of  current,  and  for  that  reason  they  are  called 
Open  Circuit  Cells.  These  cells  are  extensively  used  for  ringing  bells,  for 
setting  signals,  for  telephones,  etc.,  where  the  work  is  intermittent  and 
the  cells  have  time  to  regain  a  proper  working  condition  while  resting 
between  the  periods  of  activity. 

A  majority  of  the  cells  making  use  of  the  electrochemical  methods  of 
depolarizing,  have  no  tendency  to  polarize,  and  may  therefore  be  used 
continuously.  Such  cells  are  called  Constant  or  Closed  Circuit  Cells. 

QUESTIONS 

1.  What  are  electric  batteries? 

2.  How  is  the  energy  for  an  electric  current  obtained  in  an  electric  battery? 

3.  What  happens  when  two  dissimilar  plates  are  dipped  into  an  electrolyte? 

4.  If  the  plates  are  connected  by  a  wire,  which  one  is  attacked  by  the  electrolyte? 

5.  After  whom  was  the  volt  named? 

6.  What  did  Volta  discover? 

7.  Describe  a  voltaic  cell. 

8.  Describe  a  voltaic  pile. 

9.  Must  energy  be  continually  expended  to  keep  a  current  flowing  through  a  bat- 
tery circuit? 

10.  Which  is  the  positive  pole  of  a  cell? 

11.  What  are  the  electrodes  of  a  cell? 

12.  What  is  the  electrolyte  of  a  cell? 

13.  Is  there  any  difference  between  electricity  generated  by  a  battery  and  that  gen- 
erated by  other  means? 

14.  What  is  a  closed  circuit? 

15.  Will  electricity  flow  if  the  circuit  is  open? 

16.  Is  the  pressure  in  all  kinds  of  battery  cells  the  same? 

17.  Which  pole  is  positive  in  a  zinc-copper-sulphuric  acid  cell? 

1 8.  In  two  cells,  both  using  sulphuric  acid,  but  one  having  zinc  and  lead  elec- 
trodes and  the  other  lead  and  copper,  in  which  is  the  lead  positive?     In  which  is  it 
negative? 

19.  What  are  some  of  the  materials  used  in  making  batteries? 

20.  How  are  cells  connected  in  series? 

21.  If  10  cells,  each  of  which  produces  a  pressure  of  2  volts,  are  connected  in 
series,  what  will  be  the  total  pressure? 

22.  Does  the  size  of  the  cell  electrodes  affect  the  pressure  produced? 

23.  How  can  it  be  shown  that  the  size  of  the  electrodes  does    not    affect  the 
pressure  ? 

24.  What  causes  the  ordinary  polarization  which  occurs  in  primary  cells  ? 


ELECTRICITY  AND   MAGNETISM 


25.  What  are  the  effects  of  the  hydrogen  bubbles  that  collect  upon  the  copper 
plates  in  a  zinc-copper-sulphuric  acid  cell  ? 

26.  Why  is  the  pressure  which  the  hydrogen  tends  to  set  up  called  a  counter 
electric  pressure? 

27.  In  what  three  ways  may  depolarization  be  effected? 

28.  What  are  open  circuit  cells? 

29.  Why  can  open  circuit  cells  be  used  satisfactorily  for  ringing  electric  door  bells  ? 

43.  Mechanical  Depolarization. — The  first  method  of  depolarizing 
requires  that  the  hydrogen  bubbles  be  cleared  off  the  positive  plate  as 
fast  as  they  are  deposited  upon  it.  This  may  be  done  by  continuously 
stirring  the  liquid  or  blowing  air  into  it.  If  the  positive  plate  is  well 
roughened,  the  hydrogen  bubbles  will  not  stick  to  it  so  closely,  but  many 
will  float  off  to  the  surface  of  the  liquid  and  escape.  This  plan  was 
employed  in  a  cell  commonly  called  Smee's  cell,  which  was  used  com- 
mercially many  years  ago,  but  it  was  not  very  successful. 


FIG.  16.  —  Open  Circuit  Cell  with  FlG.  17.  —  Open  Circuit  Cell  with  Posi- 

Positive   Pole  composed  of  a  live   Pole  composed  of  a  number 

Slit  Hollow  Cylinder  of  Car-  of  Carbon   Rods   set    in    a  Circle 

bon    which    nearly    surrounds  around  the  Negative  Pole  made  of 

the  Negative  Pole  made  of  Zinc  Zinc  Rod. 
Rod. 

There  are  also  a  great  many  cells  used  for  ringing  bells,  etc.,  that 
depend  entirely  upon  the  use  of  a  large  positive  plate  surface  to  lessen 
the  rapidity  of  polarization.  It  is  evident  that  such  cells  can  only  be 
used  intermittently.  Figures  16  and  17  show  two  forms  of  these  cells. 


PRIMARY   BATTERIES 


37 


They  consist  almost  always  of  zinc  and  carbon  electrodes  immersed  in 
a  solution  of  sal  ammoniac  (the  scientific  name  for  which  is  ammonium 
chloride) .  The  carbons  are  made  with  very  large  surfaces  and  in  end- 
less variety  of  forms. 

44.  Chemical  Depolarization ;  Bichromate  Cells.  —  If  some  sub- 
stance is  added  to  the  liquid  of  the  cell  which  will  combine  with  the 
hydrogen  as  quickly  as  it  is  formed,  the  polarization  will  evidently  be 
avoided.  This  is  the  foundation  of  the  second  method  of  depolarizing. 
Various  substances  may  be  used  for  this  purpose,  but  dioxide  of  manga- 
nese, bichromate  of  pot- 
ash, bichromate  of  soda, 
chloride  of  lime  bleach- 
ing powder,  and  nitric 
acid  are  used  most 
commonly.  The  well- 
known  Bichromate  Bat- 
tery, which  is  often  used 
to  run  small  motors,  ig- 
nite the  gas  in  gas-en- 
gines, and  for  similar 
purposes,  is  a  zinc-car- 
bon battery,  with  a  liquid 
composed  of  sulphuric 
acid  in  which  bichro- 
mate of  potash  or  soda 
is  dissolved.  When  this 
cell  is  in  operation,  polar- 
ization is  prevented  by 
the  immediate  combina- 
tion, with  the  bichromate  of  potash  or  soda,  of  the  hydrogen  which  is 
liberated  from  the  sulphuric  acid.  Carbon  is  used  for  the  positive  plate 
in  this  cell  because  the  bichromate  of  potash  will  attack  and  destroy 
copper.  In  the  bichromate  battery  the  zincs  are  generally  arranged  so 
that  they  may  be  lifted  out  of  the  fluid  when  the  cells  are  not  in  use, 
because  the  fluid  eats  up  zinc  when  the  circuit  of  the  cell  is  open. 
From  this  comes  the  name  Plunge  Battery  (Fig.  18). 


FIG.  18.  —  Bichromate  Plunge  Battery. 


ELECTRICITY  AND   MAGNETISM 


The  ordinary  proportions  in  which  the  solution  for  this  type  of  bat- 
tery is  made  up  are  as  follows  :  — 

1 80  parts  of  water. 

25  parts  of  commercial  sulphuric  acid. 
1 2  parts  of  bichromate  of  potash,  or  bichromate  of  soda. 

The  crushed  bichromate  should  first  be  dissolved  in  the  water  at  boiling 
temperature,  after  which  the  acid  may  be  added  to  the  cooled  solution. 

There  are  a  large  number  of  cells  using  the  bichromate,  or  chromic 
acid,  as  depolarizers,  for  a  description  of  which  reference  should  be 
made  to  a  book  specially  describing  primary  batteries. 

45.  Chemical  Depolarization  ;  Bunsen  and  Grove  Cells.  —  When  nitric 
acid  is  used  as  a  depolarizer,  it  cannot  be  allowed  to  come  in  contact 
with  the  zinc,  which  it  attacks  vigorously  ;  consequently  it  is  confined  in 
a  porous  earthenware  cup  within  which  is  the  positive  pole  of  carbon  or 
platinum.  Figure  19  shows  such  a  cell  complete,  and  Figure  20  shows 


NEGATIVE  TERMINAL 

POSITIVE  TERMINAL 
CARBON  PLATE 


_ZINC  PLATE 


FIG.  19.  —  Bunsen  Cell. 


SULPHURIC  ACID 
^NITRIC  ACID 


FIG.  20.  —  Sectional  View  of  Bunsen 
Cell. 


the  same  cell  in  cross  section.  The  earthenware  porous  cup  is  suffi- 
cient to  prevent  the  liquids  from  mixing,  but  after  it  has  become  well 
soaked  it  does  not  present  much  resistance  to  the  passage  of  an  electric 
current.  The  cells  which  are  made  up  with  nitric  acid  for  the  depolar- 
izer are  only  useful  for  furnishing  current  for  experimental  purposes,  and 


PRIMARY   BATTERIES 


39 


TERMINAL  — 


-{-TERMINAL 


for  that  purpose  they  are  much  more  expensive  than  dynamos.     They 
.  have,  therefore,  practically  gone  out  of  use.     The  commonest  forms  of 
cells  of  this  type  are  known  as  Bunsen's 
and  Grove's  Cells.    The  action  of  the  nitric 
acid  as  a  depolarizer  is  quite  similar  to  that 
of  bichromate   of  potash,   but  it  is  more 
powerful. 

46.  Chemical  Depolarization ;  Copper 
Oxide  and  Silver  Chloride  Batteries. — 
Copper  oxide  as  a  depolarizer  was  made 
use  of  originally  by  Lalande  and  Chaperone. 
In  America,  probably  the  best-known  type 
of  cell  using  copper  oxide  is  that  called  the 
Edison-Lalande  cell.  Figure  21  exhibits 
this  cell,  in  which  Z  represents  the  zinc 
electrode,  and  C  the  positive  or  copper 
electrode  which  consists  of  a  plate  of  com- 
pressed copper  oxide.  The  liquid  used  is 
caustic  potash  or  soda  dissolved  in  water. 
A  layer  of  paraffine  oil  is  placed  over  this 

liquid  to  prevent  action  upon  it  by  the  carbonic  acid  of  the  air,  which 
would  not  only  destroy  its  effectiveness,  but  also  by  a  secondary  reaction 


FIG.  21.  —  Cell  with  Copper  Oxide 
Depolarizer. 


FIG.  22.  —  Silver  Chloride  Battery. 


ELECTRICITY   AND   MAGNETISM 


would  destroy  the  copper  oxide.  The  hydrogen  set  free  when  the  cell  is 
in  action  combines  with  the  oxygen  of  the  copper  oxide  plate,  thus  pre- 
venting polarization.  This  cell  is  used  extensively  for  medical  purposes. 

A  chloride  of  silver  battery  is  shown  in  Figure  22,  where  Z,  Zare  the 
zincs,  and  S,  S  are  the  silver  plates  upon  which  silver  chloride  has  been 
cast.  The  liquid  used  is  ammonium  chloride,  or  sal  ammoniac.  Zinc 
chloride  is  formed  at  the  zinc  plates,  and  the  free  combination  of  hydro- 
gen and  nitrogen  which  results  from  the  action  of  the  battery  forms  am- 
monium chloride  at  the  silver  plates  by  drawing  chlorine  from  the  silver 
chloride  depolarizer.  The  silver  chloride  battery  is  especially  adapted 
for  electric  testing,  and  should  not  be  required  to  furnish  large  currents. 

47.  Chemical  Depolarization  ;  Leclanche  Cells.  —  When  dioxide  of 
manganese  is  used  as  a  depolarizer,  it  is  generally  broken  up  into  small 
lumps  and  put  into  a  porous  cup  surrounding  a  positive  plate  of  carbon. 


CARBON  PLAT 
RUBBER  BAND^" 


RUBBER  BAND—"*T 


MANGANESE  DIOXIDE' 
BRICKS 


FIG.  23.  —  Leclanche  Cell  with  Porous 
Cup. 


FlG.  24.  —  Carbon  Plate  with  Dioxide 
Prisms,  Zinc,  and  Cover  of  "  Prism  " 
Leclanche  Cell. 


When  sal  ammoniac  dissolved  in  water  is  used  as  the  liquid  in  this  form 
of  cell,  it  makes  the  familiar  Leclanch£  battery  (Fig.  23),  which  is  used 
so  frequently  in  ringing  door  bells  and  in  doing  similar  service.  Some- 


PRIMARY   BATTERIES  41 

/ 

times  the  dioxide  of  manganese  is  pulverized  and  mixed  with  shellac, 
after  which  it  is  pressed  into  small  bricks,  which  are  placed  upon  either 
side  of  the  carbon  positive  plate  (Fig.  24),  as  in  the  "  prism  "  Leclanche" 
battery.  The  depolarizing  effect  of  dioxide  of  manganese  is  not  suffi- 
ciently powerful  to  prevent  a  cell  from  becoming  polarized  if  used  con- 
stantly. Consequently,  Leclanche"  cells  are  only  satisfactory  in  service 
which  is  intermittent,  like  ringing  door  bells,  where  the  circuit  is  open  a 
considerable  part  of  the  time  and  the  battery  rests  without  chemical 
action.  Leclanche"  cells  are  called  open  circuit  cells  on  account  of  the 
small  chemical  action  which  goes  on  in  them  when  the  circuit  is  open, 
and  because  they  are  not  satisfactory  in  continuous  service.1 

48.  Electrochemical  Depolarization;  Daniell's  Battery. — The  third 
method  of  depolarizing  introduces  more  complicated  chemical  reactions, 
but  of  these  we  need  not  give  much  detail.  Through  the  use  of  this 
method  cells  are  constructed  which  give  excellent  results  in  continuous 
service,  and  which  are,  therefore,  called  closed  circuit  cells.2  One  of 
these  is  probably  the  most  commonly  used  of  any  form  of  battery.  This 
is  the  ordinary  Gravity  Battery,  or  copper  sulphate  battery,  which  is  so 
much  used  in  telegraphy. 

In  the  original  Daniell's  cell  from  which  the  gravity  cell  came,  the 
active  liquid  is  dilute  sulphuric  acid  in  which  is  immersed  the  zinc  or 
negative  plate.  The  copper  plate  is  immersed  in  a  depolarizing  solution 
of  ordinary  copper  sulphate,  or  blue  vitriol  (sometimes  called  bluestone). 
The  two  solutions  are  separated  by  a  porous  cup.  In  general  terms  the 
chemical  action  which  occurs  when  the  battery  is  working  is  as  follows  : 
The  sulphuric  acid  attacks  the  zinc,  and  sulphate  of  zinc  is  formed.  At 
the  same  time  hydrogen  is  liberated  from  the  sulphuric  acid  and  goes 
toward  the  copper  plate,  where  it  would  be  deposited  if  it  were  not  for 
the  copper  sulphate  which  surrounds  the  copper  plate.  When  the  hydro- 
gen gets  into  the  copper  sulphate  solution,  it  goes  into  combination,  and 
copper  is  separated  from  the  solution  and  deposited  upon  the  copper 
plate,  which  is  therefore  kept  bright  and  in  good  working  condition. 

During  the  operation  of  the  cell  the  chemical  action  which  has  been 
briefly  explained  causes  a  change  in  the  character  of  the  solutions.  The 
sulphuric  acid  changes  to  a  solution  of  sulphate  of  zinc,  and  the  copper 
1  See,  also,  Article  42.  2  See,  also,  Article  42. 


ELECTRICITY   AND    MAGNETISM 


COPPER  TERMINAL 


FIG.  25.^  Daniell's  Cell. 


sulphate  changes  to  sulphuric  acid.     If  the  sulphuric  acid  is  replaced  by 
a  dilute  or  weak  solution  of  zinc  sulphate,  a  current  is  set  up,  as  before, 

ZINC  TERMINAL  anc*  tne  cnermcal  action  is  similar,  but 
the  copper  sulphate  is  converted  into 
zinc  sulphate.  In  order  that  the  de- 
polarizing action  may  continue  during 
the  Life  of  the  cell,  the  strength  of  the 
copper  sulphate  solution  must  be  kept 
up.  This  is  done  by  putting  crystals  of 
copper  sulphate  or  blue  vitriol  into  the 
cell  so  that  they  may  be  dissolved. 
Figure  25  shows  a  cell  of  this  battery 
in  its  original  form,  in  which  it  is  called 
Daniell's  battery.  In  the  figure,  the  zinc 
plate  is  shown  within  the  porous  cup  at 
the  right  hand  of  the  battery  jar,  and  the 
copper  plate  is  at  the  left  hand  of  the 
jar.  Alongside  of  the  copper  plate  is  a  perforated  copper  cage  in  which 
may  be  put  the  copper  sulphate  for  renewing  the  solution. 

49.  Electrochemical  Depolarization;  Gravity  Cell. — The  sulphuric 
acid  or  zinc  sulphate  solution  of  this  modification  of  the  Daniell's  cell  is 
ordinarily  much  Diluted  or  weakened  by 
water,  while  the  copper  sulphate  solution 
is  kept  quite  strong  or  Saturated.  When 
in  this  condition  the  solution  of  zinc  sul- 
phate is  lighter  than  the  other,  and  will 
float  upon  it,  just  as  oil  floats  on  water. 
Consequently,  if  the  copper  plate,  sur- 
rounded by  the  solution  of  copper  sul- 
phate, is  placed  in  the  bottom  of  a  battery 
jar,  a  weak  solution  of  zinc  sulphate  or 
sulphuric  acid  may  be  carefully  poured  on 
top,  and  the  solutions  will  mix  only  very 
slowly.  The  zinc  may  be  hung  from  the 
top  of  the  jar  in  the  upper  solution  (Fig.  26).  This  constitutes  the 
gravity  battery,  so  called  because  the  solutions  are  separated  by 


Lfl  OF 

COPPER  SULPHATE 


FIG.  26.  — Gravity  Cell. 


PRIMARY   BATTERIES 


43 


gravity   through   the   difference    in    their    densities,   instead   of    by   a 
porous  cup. 

In  setting  up  such  a  cell  it  is  usual  to  put  the  copper  in  the  bottom  of 
the  jar  surrounded  by  crystals  of  copper  sulphate.  The  jar  is  then  filled 
with  water  to  near  its  top,  and  the  zinc  is  immersed  in  the  upper  part  of 
the  liquid.  The  cell  may  be  placed  on  Short  Circuit  for  a  time,  and  it 
will  work  itself  into  good  operating  condition,  or  a  little  sulphuric  acid 
or  zinc  sulphate  solution  may  be  carefully  poured  into  the  water,  and  the 
cell  will  at  once  be  in  condition. 

If  a  gravity  cell  is  allowed  to  stand  upon  open  circuit,  the  two  solutions 
will  slowly  mix  by  Diffusion.  When  any  of  the  copper  sulphate  solution 
reaches  the  zinc,  a  black  deposit  of  oxide  of  copper  is  made  on  it.  This 
puts  the  cell  in  such  condition  that  it  will  not  work  satisfactorily  until  the 
zinc  has  been  cleaned.  When  the  cell  is  in  operation,  the  copper 
sulphate  is  changed  into  zinc  sulphate  so  rapidly  that  it  gets  no  chance 
to  mix  with  the  latter.  A  gravity  battery,  therefore,  is  only  satisfactory 
in  the  service  which  keeps  it  constantly  working.  For  this  reason  and 
also  because  it  does  not  polarize  in  the  least,  it  is  called  a  closed 
circuit  battery. 

There  are  various  other  types  of  bat- 
teries in  which  the  third  method  of  de- 
polarizing is  used,  but  which  are  not  in 
sufficiently  general  use  to  make  their  de- 
scription desirable  here. 

50.  Dry  Batteries.  —  During  the  last  few 
years,  what  are  called  dry  battery  cells  have 
been  coming  into  quite  extensive  use  for 
intermittent  work.  These  cells  usually  have 
zinc  and  carbon  electrodes,  while  the  elec- 
trolyte is  in  the  form  of  a  paste  instead 
of  being  liquid  as  in  the  ordinary  cells. 
Probably  the  oldest  form  of  dry  cell  is  one 
invented  by  Gassner,  and  shown  in  Figure 
27.  The  zinc  is  made  in  the  form  of  a  deep 
cup  and  is  the  containing  vessel.  In  the 


middle,  and  occupying  probably  one-half 


FIG.  27.  — Dry  Cell,  Showing  the 
Filling. 


44  ELECTRICITY  AND   MAGNETISM 

of  the  space  inside  the  zinc  cup,  is  the  carbon  electrode.  Between 
the  zinc  and  the  carbon  a  paste  is  placed  which  is  composed  of  the 
following:  "Oxide  of  zinc,  one  part,  by  weight;  sal  ammoniac,  one 
part,  by  weight  ;  plaster,  three  parts,  by  weight ;  chloride  of  zinc,  one 
part,  by  weight;  water,  two  parts,  by  weight."  The  oxide  of  zinc  is 
used  for  making  the  paste  porous,  which  permits  the  escape  or  combi- 
nation of  gases  and  hence  lessens  polarization.  This  cell  has  a  pressure 
of  1.3  volts,  and  will  polarize  rapidly  if  kept  long  in  circuit. 

A  great  many  dry  batteries  of  different  compositions  are  now  on  the 
market,  and  they  have  proved  themselves  very  convenient  and  effective. 
It  is  interesting  to  know  that  the  resort  to  dry  batteries  is  to^a  certain 
extent  a  return  toward  Volta's  Pile. 

51.  Local  Action.  —  In  nearly  all  battery  cells  some  chemical  action, 
by  which  the  zinc  is  wasted,  goes  on  when  the  circuit  is  open.   This  may 
also  proceed  while  the  circuit  is  closed  without  adding  to  the  useful 
current  in  the  cell.    Such  wasteful  chemical  action  is  called  Local  Action. 
It  is  usually  caused  by  bits  of  impurity  or  differences  of  composition  on 
the  surface  of  the  zinc,  which  form  little  local  cells  with  the  other  por- 
tions of  the  zinc,  thus  continuously  eating  it  away  in  spots.    This  action 
may  be  seen  by  placing  a  piece  of  ordinary  commercial  zinc  in  dilute 
sulphuric  acid,  when  a  chemical  action  takes  place  which  is  similar  to 
that  described  above  in  the  case  of  a  cell  generating  a  current,  and  the 
zinc  is  dissolved ;  while,  if  the  zinc  is  chemically  pure  (that  is,  does  not 
contain  any  impurities),  the  action  does  not  occur. 

Local  action  is  also  caused  in  some  cells  by  differences  in  the  density  of 
the  liquid  at  various  parts  of  the  cell.  In  this  case,  the  zinc  near  the  top 
of  the  liquid  is  ordinarily  wasted  away,  and  it  may  be  entirely  eaten  off. 

52.  Amalgamation.  —  Local    action    caused   by   impurities   may  be 
largely  avoided  by  Amalgamating  the  zinc,  that  is,  by  Alloying  its  surface 
with  mercury.     For  this  purpose  the  zinc  is  cleaned  by  dipping  into  a 
dilute  acid  solution,  and  it  is  then  rubbed  with  mercury,  which  makes  a 
pasty  alloy  on  its  surface.     The  impurities  in  the  zinc  do  not  readily 
form  an  amalgam  with  the  mercury  and  are  therefore  covered  up,  while 
pure  zinc  is  brought  to  the  surface.     As  the  zinc  is  eaten  away,  the  mer- 
cury remains  and  combines  with  the  zinc  below,  thus  keeping  the  zinc 
plate  in  good  condition  until  it  is  practically  all  used. 


f 

s  LOCAL  ACTION  45 

Zinc  plates,  or  Zincs  for  batteries,  are  also  sometimes  cast  with  a  small 
percentage  of  mercury  in  their  composition,  which  is  intended  to  take 
the  place  of  amalgamation.  The  mercury  seems  to  cover  all  im- 
purities and  to  present  only  pure  zinc  at  the  surface. 

53.  Amount  of  Chemical  Action  in  a  Cell.  —  The  amount  of  metal, 
such  as  zinc,  usefully  consumed  in  a  cell  depends  directly  upon  the 
number  of  coulombs  of  electricity  which  are  permitted  to  pass  through 
the  electrolyte.     The  amount  of  hydrogen  gas,  copper,  or  other  metals 
liberated  from  the  liquid  also  depends  upon  the  number  of  coulombs  of 
electricity  which  pass  through  the  cell.    This  may  be  stated  as  a  general 
law  of  electrochemical  action  :    the  amount  of  chemical  action  in  a  cell 
depends  directly  upon  the  amount  of  electricity  which  passes  through  it,  and 
therefore  the  chemical  action  is  the  same  in  all  cells  of  a  number  connected 
in  series,  since  the  same  amount  of  current  will  flow  through  them  all. 

The  weight  of  a  metal  in  grammes  (metric  measure)  which  is  dissolved 
or  deposited  when  one  coulomb  of  electricity  passes  through  a  cell,  is 
called  the  Electrochemical  Equivalent  of  the  metal.  A  table  which 
shows  the  electrochemical  equivalents  of  various  chemical  elements  is 
included  in  Article  65. 

54.  Value  of  Zinc  as  a  Fuel.  —  Electric  batteries  in  which  a  metal  is  \ 
directly  consumed  by  chemical  action  for  the  generation  of  an  electric  j 
current  are  called   Primary  Batteries.     In  nearly  all  primary  batteries  I 
the  metal   which  is  consumed   is  zinc.      The  law  of  electrochemical 
action  already  stated  shows  that  no  current  can  be  produced  without  an 
equivalent  consumption  of  metal,  just  as  an  appreciable  amount  of  heat 
cannot  be  given  out  from  a  fire  without  an  appreciable  consumption  of 
coal  or  wood. 

The  consumption  of  zinc  in  a  battery  to  furnish  electrical  energy  in 
the  form  of  an  electric  current  is  similar  to  the  burning  of  coal  under  a 
boiler  to  furnish  steam  power.  It  can  be  readily  seen  that  zinc  makes 
an  expensive  fuel,  even  though  the  consumption  of  a  pound  of  zinc 
in  a  battery  produces  several  times  as  much  energy  as  is  produced  by 
the  combustion  of  a  pound  of  coal  in  the  furnace  of  a  boiler  ;  and  bat- 
teries in  which  zinc  is  consumed  cannot  be  used  commercially  to  furnish 
electricity  where  currents  of  great  magnitude  are  required,  as  in 
electric  lighting. 


46  ELECTRICITY  AND   MAGNETISM 

For  such  purposes  the  battery  can  never  compete  with  the  dynamo 
driven  by  a  steam  engine,  unless  a  cell  is  invented  in  which  coal  may  be 
economically  consumed  in  the  place  of  zinc  so  that  the  heat  due  to 
the  combustion  of  coal  may  be  thus  directly  transferred  into  electrical 
energy,  or  a  cyclic  system  is  commercially  developed  in  which  the  metal 
dissolved  in  the  battery  may  be  recovered  through  the  action  of  electric 
currents  generated  by  water  power.  If  this  is  ever  done,  the  electric 
battery  will  displace  the  steam  boiler  and  engine,  but  batteries  in 
which  zinc  is  consumed  can  never  economically  furnish  current  for 
light  and  power. 

55.  Where  Batteries  are  Valuable.  —  In  many  domestic  operations, 
such  as  ringing  electric  bells,  regulating  dampers,  etc.,  primary  batteries 
hold  an  important  place.     In  telegraphy  and  telephony,  and  other  com- 
mercial applications  of  electricity  and   magnetism  on  a  large  scale  in 
which  comparatively  weak  currents  are  required,  they  are  used  in  great 
numbers.      They   are    also    used    in    electrotherapeutics    and    similar 
applications. 

For  many  domestic  purposes  the  work  required  of  a  battery  is  inter- 
mittent, and  so  small  that  a  cell  of  constant  electromotive  force  is  not 
required.  Consequently  many  batteries  are  made  of  simple  zinc-carbon 
cells  in  which  the  liquid  is  a  solution  of  sal  ammoniac.  These  cells  are 
like  Leclanche"  cells  without  the  dioxide  of  manganese  depolarizer. 
The  carbon  plate  is  generally  made  with  a  large  surface,  as  illustrated  in 
Figures  16  and  17,  so  that  the  polarization  is  not  very  rapid. 

56.  Storage  Batteries.  —  If  a  gravity  cell  is  worked  until  its  solution 
contains  plenty  of  zinc  sulphate,  and  a  current  is  then  passed  through  it 
from  the  copper  plate  to  the  zinc  plate,  metallic  zinc  will  be  deposited 
on  the  zinc  plate  by  the  chemical  action  due  to  the  current.     The  current 
which  separates   the  zinc  from  the  liquid  is  passed    through    the    cell 
against  the  electric  pressure  naturally  developed  by  the  cell,  and  energy 
must  be  expended  in  order  that  the  current  may  flow.     This  energy  is 
stored  up  during  the  process,  in  the  deposited  zinc,  and  may  be  returned 
when  the  zinc  is  again  dissolved  through  the  operation  of  the  battery 
in  the  ordinary  manner. 

Alternate  Discharging  of  the  battery  by  taking  current,  and  conse- 
quently energy,  from  it  through  the  consumption  of  zinc,  and  then  again 


STORAGE   BATTERIES  47 

Charging  it  by  expending  energy  in  the  cell  by  sending  current  into  it 
and  depositing  zinc,  may  be  kept  up  indefinitely.  Each  time  the  cell 
is  discharged,  it  gives  out  as  much  energy  through  the  consumption  of 
its  zinc,  as  was  given  to  it  in  depositing  the  same  amount  of  zinc. 
There  are 'certain  deductions  of  energy  from  the  external  electrical  cir- 
cuit, however,  which  are  inevitably  linked  with  these  operations,  so  that 
it  would  be  an  error  for  the  reader  to  assurne  that  the  Efficiency  of  the 
cell  is  one  hundred  per  cent. 

A  battery  in  which  energy  may  be  stored  through  the  forced  chemi- 
cal action  called  charging,  and  from  which  this  energy  may  be  then 
withdrawn  through  the  natural  action  of  the  cells,  is  called  a  Storage 
Battery.  Storage  batteries  are  also  called  Accumulators  or  Secondary 
Batteries. 

Commercial  storage  cells  are  usually  made  with  lead  plates  immersed 
in  dilute  sulphuric  acid.  When  a  cell  of  this  type  is  fully  charged,  one 
of  the  plates,  i.e.  the  negative  one,  is  of  a  grayish  color  and  is  practi- 
cally pure  lead  with  a  surface  in  a  more  or  less  spongy  state.  The  other 
plate  is  of  a  brownish  or  plum  color,  due  to  the  fact  that  its  surface  is 
covered  with  a  layer  of  the  brownish  oxide  of  lead,  or,  as  it  is  called, 
peroxide  of  lead.  When  the  circuit  is  closed,  a  current  flows. 

The  flow  of  current  within  the  cell  is  from  the  gray  plate  toward  the 
reddish  plate,  when  the  cell  is  being  discharged.  The  electrical  pressure 
of  the  cell  is  set  up  by  chemical  reactions  between  the  plates  and  the  sul- 
phuric acid,  which,  when  the  current  flows,  change  the  lead  oxide  on 
the  red,  oxidized  plate  into  sulphate  of  lead,  and  at  the  same  time  also 
change  the  lead  of  the  grey  plate  into  sulphate  of  lead.  The  cell  con- 
tinues to  give  out  current  until  the  surfaces  of  the  plates  become,  to  a 
more  or  less  equal  degree,  changed  into  sulphate  of  lead,  and  then 
the  pressure  falls  rapidly,  as  it  will  be  remembered  that  similar  plates 
in  a  solution  do  not  exhibit  an  electromotive  force.  Under  this  condition 
the  battery  is  said  to  be  fully  discharged,  and  it  can  be  charged  up  again 
by  passing  current  through  it  in  the  opposite  direction. 

The  charging  process  is  one  in  which  the  negative  plate  is  restored  to 
its  plain  leaden  condition,  and  the  positive  plate  becomes  oxidized  again. 
When  the  negative  plate  has  been  reduced  to  practically  plain  lead  with 
a  spongy  surface,  the  cell  is  said  to  be  fully  charged.  In  actual  work  a 


48 


ELECTRICITY  AND   MAGNETISM 


cell  is  never  allowed  to  become  entirely  discharged,  as  complete  dis- 
charge is  very  likely  to  result  in  great  injury  to  the  plates. 

57.  Construction  of  Lead  Cells.  —  Since  the  chemical  action  which 
goes  on  in  these  storage  cells  during  charging  and  discharging  roughly 
consists  in  transferring  oxygen,  which  exists  in  the  oxide  of  sulphate  of 
lead  on  the  plates,  from  one  plate  to  the  other,  it  is  desirable  that  the 
plates  be  capable  of  holding  a  large  amount  of  oxide  of  lead  in  order 
that  the  cells  may  be  of  large  capacity ;  and  they  are  therefore  made 
with  corrugations  or  perforations  in  which  the  oxide  may  be  fixed.  The 
perforated  plates  are  called  Grids. 

Sometimes  the  plates  are  made  up  for  use  by  filling  the  perforations 
in  the  grids  with  a  paste  consisting  of  lead  oxide  moistened  with  sul- 
phuric acid.  This  process  is  called  Pasting ;  and  plates  made  up  thus 
are  often  called  Pasted  plates,  or  Faure  plates  after  the  name  of  the 
inventor  of  the  method.  Sometimes  the  oxide  is  formed  by  frequent 

charging  and  discharging  of  the  cell. 
This  process  is  called  Forming,  and 
plates  of  this  kind  are  called  Plante" 
plates,  after  the  original  inventor  of 
the  lead  plate  storage  battery,  who 
used  this  method.  In  the  process 
of  pasting  it  is  usual  to  use  the  yel- 
low oxide  of  lead,  which  is  com- 
monly known  as  litharge,  to  paste 
on  the  negative  plates,  and  the  red 
oxide,  which  is  commonly  known  as 
minium,  to  paste  on  the  positive 
plates.  These  turn  into  plain  lead  on 
the  negative  and  peroxide  of  lead  on 
the  positive  plates,  during  the  process 
of  charging  the  battery.  Some  manu- 
facturers paste  both  plates  with  sul- 
FIG.  28.  — Lead  Plate  Storage  Cell.  phate  of  lead  moistened  by  sul- 
phuric acid,  instead  of  using  the 

oxides  of  lead.      The  sulphate  of  lead  is  a  white  powdery  substance 
which  can  be  bought  of  dealers  in  chemicals. 


ELECTRIC   BATTERIES  49 

Figure  28  shows  a  lead  plate  storage  cell  in  a  glass  jar.  In  place  of 
the  jar  it  is  not  unusual  to  use  a  rubber  cell,  or  a  wooden  box  lined  with 
rubber  or  lead.  In  order  that  the  cell  may  have  a  capacity  for  a  large 
current,  a  number  of  positive  and  negative  plates  are  put  alternately  in 
one  jar  and  are  connected  in  Parallel — that  is,  the  plates  are  connected 
so  that  the  current  capacity  of  the  cell  is  equal  to  the  sum  of  the  capaci- 
ties of  the  various  plates,  but  the  pressure  of  the  cell  is  the  same  as  that 
of  a  cell  made  up  of  a  single  pair  of  the  plates. 

The  positive  plates  of  a  lead  plate  storage  battery  usually  have  a 
brownish  color,  and  the  negative  plates  a  grayish  color.  The  electrical 
pressure  produced  by  a  lead  plate  cell  generally  varies  between  1.8  and 
2.2  volts  at  different  conditions  of  the  charge.  The  higher  value  occurs 
in  a  fully  charged  cell,  and  the  pressure  falls  as  the  cell  discharges. 

58.  Other  Storage  Cells.  —  Commercial  storage  batteries  are  made 
with  other  liquids  than  sulphuric  acid  and  other  than  lead  plates,  but 
they  cannot  be  given  consideration  here,  as  they  have  not  come  into 
common  use. 

QUESTIONS 

1.  How  may  depolarization  be  effected  by  mechanical  means? 

2.  Describe  a  bichromate  cell. 

3.  Which  method  of  depolarization  is  used  in  the  bichromate  cell? 

4.  How  does  depolarization  proceed  in  a  bichromate  cell? 

5.  Why  must  the  zinc  electrodes  be  withdrawn  from  the  electrolyte  of  a  bichro- 
mate battery  when  it  is  not  in  use? 

6.  Why  are  porous  cups  used  in  the  Bunsen  and  Grove  cells? 

7.  Describe  a  copper  oxide  cell. 

8.  What  causes  depolarization  in  a  copper  oxide  cell? 

9.  Describe  a  silver  chloride  cell. 

10.  Describe  a  Leclanche  cell. 

11.  What  is  the  depolarizer  in  a  Leclanche  cell? 

12.  What  service  is  the  Leclanche  cell  fitted  for?     Why? 

13.  How  is  a  Daniell  cell  constructed  and  of  what  materials? 

14.  What  method  is  used  in  depolarizing  a  Daniell  cell? 

15.  Is  the  Daniell  cell  an  open  or  closed  circuit  cell? 

1 6.  What  is  the  chemical  action  in  a  Daniell  cell? 

17.  Why  must  blue  vitriol  be  occasionally  added  to  a  Daniell  cell? 

1 8.  Why  is  a  porous  cup  used  in  a  Daniell  cell? 

19.  How  does  the  gravity  cell  differ  from  the  Daniell  cell? 

E 


5O  ELECTRICITY  AND   MAGNETISM 

20.  Why  is  the  solution  of  zinc  sulphate  diluted  and  the  copper  sulphate  made 
strong  in  a  gravity  cell? 

21.  Why  is  a  gravity  battery  short  circuited  for  a  time  when  it  is  first  set  up? 

22.  What  is  required  to  prevent  diffusion  in  a  gravity  cell  ?     W7hy? 

23.  What  is  the  effect  of  the  diffusion  of  the  fluids  in  a  gravity  cell? 

24.  What  are  dry  cells,  and  how  are  they  constructed? 

25.  What  is  local  action  in  a  cell? 

26.  How  is  local  action  caused? 

27.  What  is  amalgamation? 

28.  How  does  the  mercury  apparently  act  when  an  amalgamated  zinc  is  placed  in 
a  cell? 

29.  How  may  zinc  be  amalgamated? 

30.  On  what  does  the  amount  of  chemical  action  in  a  cell  depend? 

31.  State  the  general  law  for  electrochemical  action. 

32.  How  much  more  copper  will  be  deposited  in  one  minute  in  a  Daniell  cell 
through  which  a  current  of  five  amperes  flows  than  will  be  deposited  in  a  similar  cell 
through  which  one  ampere  flows? 

33.  If  several  gravity  cells   of  different  sizes  were  connected  in  circuit  in  series, 
would  equal  amounts  of  copper  be  deposited  in  each? 

34.  What  are  electrochemical  equivalents? 

35.  What  are  primary  batteries? 

36.  Why  cannot  primary  batteries  in  which  zinc  is  consumed  be  used  economi- 
cally to  furnish  current  for  lighting  and  power? 

37.  Name  a  number  of  uses  to  which  primary  batteries  may  be  put. 

38.  \Vhat  is  a  storage  battery? 

39.  What  happens  in  a  gravity  cell  when  current  is  forced  through  it  from  copper 
to  zinc? 

40.  Of  what  are  commercial  storage  batteries  made? 

41.  What  is  the  character  of  the  positive  plates  in  a  storage  battery  when  it  is 
charged?      When  it  is  discharged? 

42.  What  is  the  character  of  the  negative  plates  in  a  storage  battery  when  it  is 
charged  ?     When  it  is  discharged? 

43.  Explain  the  action  in  a  storage  battery  when  it  is  discharging,  and  when  it  is 
charging. 

44.  How  are  lead  plate  cells  constructed  ? 

45.  What  are  grids? 

46.  \Vhat  is  the  difference  between  a  pasted  and  a  formed  plate? 

47.  What  is  meant  by  the  statement  that  the  sets  of  negative  plates  (also  the  posi- 
tive) in  a  cell  are  "  connected  in  parallel  ?  " 

48.  What  are  the  pressures  of  a  lead  storage  cell? 


CHAPTER   V 

ELECTROLYSIS 

59.  Electrolytic    Conductors.  —  An    electric   current   seems   to  flow 
through  some  liquids  in  a  different  way  from  that  in  which  it  flows 
through  solid  conductors.     In  fact,  liquids  may  be  divided  into  three 
classes  on  the  ground  of  their  action  when  subjected  to  the  effect  of  an 
electric  pressure :  — 

1.  Those  which  appear  to  be  insulators  of  a  high  grade,  such  as 
paraffine  oil,  turpentine,  etc. 

2.  Those'  which   conduct   like   solids,   without   apparent   chemica 
action,  such  as  mercury,  metals  in  a  melted  condition,  etc. 

3.  Those  in  which  chemical  decomposition  occurs  when  a  curren 
flows  through  them,  such  as  solutions  of  acids  or  salts  of  the  metals 
and  some  melted  compounds. 

Liquids  of  the  latter  class  are  called  Electrolytes,  and  the  process  of 
their  decomposition  by  electrochemical  action  is  called  Electrolysis.  A 
cell  in  which  electrolysis  is  carried  on  is  generally  called  an  Electrolytic 
Cell,  or  when  the  electrochemical  action  is  used  to  determine  the 
strength  of  the  current  flowing  through  the  cell,  it  is  called  a  Voltameter, 
as  will  be  explained  later.  The  plates  of  an  electrolytic  cell  are  called 
Electrodes.  The  Positive  Electrode  (the  one  at  which  the  current 
enters)  is  often  called  the  Anode,  and  the  Negative  Electrode,  the 
Cathode.  The  products  of  the  electrolysis  are  often  called  Ions. 

60.  Action  in  Electrolytic  Cells. — We  will,  for  a  moment,  consider 
the  elementary  performance  of  an  electrolytic  cell  in  which  the  elec- 
trodes are  copper  plates  and  the  electrolyte  is  a  solution  of  a  copper 
salt.     The  commonest  salts  of  copper  are  the  sulphate  of  copper,  the 
nitrate  of  copper,  the  chloride  of  copper,  the  carbonate  of  copper,  and 
the  sulphide  of  copper.     Any  salt  of  a  metal  is  a  chemical  combination 


ELECTRICITY   AND   MAGNETISM 


formed  by  the  action  of  an  acid  on  the  metal.  Thus,  sulphate  of  copper 
is  a  combination  which  may  be  formed  by  the  action  of  sulphuric  acid 
on  copper ;  and  nitrate  of  copper  may  be  formed  by  the  action  of  nitric 
acid  on  copper.  Sulphuric  acid  is  a  chemical  compound  of  hydrogen 
with  sulphur  and  oxygen,  —  the  sulphur  and  oxygen  in  this  case  forming 
what  is  called  an  acid  radical.  The  radical  of  sulphuric  acid  has  a 
greater  chemical  attraction,  or  Affinity,  for  copper  than  for  hydrogen, 
especially  when  the  acid  is  hot.  Consequently,  when  copper  is  immersed 
in  hot  sulphuric  acid  the  copper  is  attacked  and  dissolved,  during  which 
process  it  combines  with  the  acid  radical  of  a  portion  of  the  acid  and 
forms  sulphate  of  copper.  The  copper  sulphate  stays  in  the  solution 
unless  means  are  taken  to  cause  it  to  crystallize  out. 

The  crystallized  copper  sulphate,  or  blue  vitriol,  as  it  is  often  called, 
may  be  bought  at  any  drug  store.  It  is  in  lumps  of  blue  crystals  which 

readily  dissolve  in  water. 

If  an  electrolytic  cell  is  made  up  by 
placing  copper  plates  in  a  vessel  con- 
taining a  solution  of  copper  sulphate 
in  water  (Fig.  29),  and  an  electric 
current  from  a  battery  is  sent  through 
the  cell,  the  copper  sulphate  becomes 
decomposed  and  metallic  copper  is 
deposited  on  the  cathode.  The  acid 
radical,  corresponding  in  equivalent 
amount  to  the  copper  which  is  de- 
posited from  the  copper  sulphate, 
makes  its  appearance  at  the  anode, 
where  its  chemical  activity  causes  it  to 
combine  with  the  copper  of  the  anode, 
which  it  gradually  eats  away.  It  is  thus  seen  that  the  anode  is  gradually 
eaten  away  or  dissolved  in  the  solution,  while  the  cathode  grows  from 
the  deposited  copper. 

This  is  the  way  copper  plating  is  done,1  though  the  cathode  to  be 
plated  is  not  usually  made  of  copper,  and  it  may,  indeed,  be  of  any  con- 
ducting material.  During  the  process  of  plating,  the  cathode  has 

1  Article  365. 


UJ 

>,      0 

J 

I 

o    ^ 

0     ' 

ELECTROLYTE 

z    ? 

/     o: 

SULPHATE  OF  COPPER 

£  ; 

u 

- 

*•             tOr 

- 

o    ; 

FlG.  29. —  Electrolytic  Cell  \\ith 
Copper  Electrodes  and  Electro- 
lyte of  Copper  Sulphate. 


ELECTROLYSIS  53 

deposited  upon  it  a  covering  of  metal,  while  an  equivalent  amount  of 
the  metal  is  dissolved  into  the  solution  from  the  anode. 

61.  Action  in  Electrolytic  Cells,  Continued.  —  In  a  similar  manner, 
other  salts  of  copper  and  the  various  salts  of  other  metals  may  be  Elec- 
trolyzed.    It  is  even  possible  to  electrolyze  liquids  which  do  not  contain 
salts  in  solution,  as  in  the  case  of  the  water  voltameter,  which  will  be 
more  fully  described  later.1     Here  the  electrodes  are  of  platinum  and 
are,  therefore,  not  dissolved,  but  the  water  is  decomposed   into   its 
constituent  parts,  which  are  hydrogen  and  oxygen,  and  these  are  given 
off  in  the  shape  of  bubbles  at  the  cathode  and  anode  respectively. 

This  action  in  using  up  the  electrolyte  itself  is  somewhat  similar  to  the 
action  that  would  have  taken  place  in  the  copper  solution  spoken  of 
above,  if  platinum  electrodes  had  been  used.  In  such  a  case  the  copper 
is,  as  before,  deposited  upon  the  cathode ;  but  the  acid  radical  cannot 
now  act  upon  the  anode,  which  is  incorrodible,  so  that  it  attacks  the 
water  in  the  solution,  takes  hydrogen  from  the  water  to  make  sulphuric 
acid,  and  thereby  sets  oxygen  free,  which  gathers  upon  the  anode  in  bubbles 
and  escapes.  This  process  continues  until  all  the  copper  is  extracted, 
after  which  the  reactions  become  the  same  as  those  explained  later  for 
acidulated  water.  It  is  seen  that  the  electrolyte  in  this  case  is  decom- 
posed, but  that  the  electrodes  are  unaffected.  In  such  manner  it  is 
possible  to  make  chemical  combinations  under  the  influence  of  the 
electric  current  which  are  of  great  value  to  commerce. 

The  metal  part,  or  its  equivalent  in  hydrogen,  of  an  electrolyzed  solu- 
tion is  always  deposited  upon  or  escapes  at  the  cathode  ;  that  is,  it  appears 
to  travel  with  the  current  to  the  electrode  where  the  current  leaves  the 
cell.  The  acid  radical,  or  its  equivalent  in  oxygen,  appears  to  travel  against 
the  direction  of  the  current  and  is  deposited  at  the  anode,  where  it  may 
either  be  given  off  as  gas,  or  by  combining  with  the  anode,  it  may  cause 
the  latter  to  be  dissolved. 

62.  Faraday's  Laws.  —  During  the  years    1833  and    1834  Faraday 
occupied   his   matchless  intellect   in   experimentally   investigating  the 
decomposition  of  electrolytes,  such  as  has  been  described  in  the  pre- 
ceding  article,    by   the  effect  of  a   current  of  electricity.     An  exact 
knowledge  of  the  more  important  conditions  of  electrolysis  (which  pre- 

1  Articles  66  and  156. 


54  ELECTRICITY   AND    MAGNETISM 

viously  was  an  almost  unexplored  field  of  phenomena)  resulted  from 
these  investigations,  and  Faraday  laid  down  his  principal  deductions  in 
two  laws,  which  may  be  stated  as  follows  :  — 

1.  When  an  electric  current  passes  through  an  electrolyte,  the  quan- 
tity of  the  liquid  decomposed  depends  upon  tiie  amount  of  electricity,  meas- 
ured in  coulombs,  that  passes   through  it,  and  is   independent  of  the 
size  of  the  electrodes  and  the  electrical  pressure  applied  to  the  cell.     The 
amount,  of  chemical  action  is  therefore  an  exact  measure  of  the  number 
of  coulombs  of  electricity  passed  through  the  electrolyte. 

2.  When  equal  quantities  of  electricity  are  passed  through   different 
electrolytes,  equivalent  quantities  of  the  electrolytes  are  decomposed. 

This  second  law  was  stated  by  Faraday  thus  :  —  "If  the  same  quan- 
tity of  electricity  passes  through  different  electrolytes,  the  masses  of  dif- 
ferent ions  liberated  at  the  electrodes  are  proportional  to  their  chemical 
equivalents." 

63.  Chemical   Equivalents.  —  The    phrase    "chemical   equivalents" 
used  by  Faraday  means  the  relative  combining  proportions,  or  equiva- 
lent quantities  in  chemical  combinations,  of  the  various  ions.     When 
chemical  elements  combine  with  each  other  to  form  compounds,  they 
are  supposed  to  always  join  the  combinations  in  certain  fixed  propor- 
tions, which  are  characteristic  of  the  individual  elements.     Thus,  if  a 
quantity  of  silver  weighing  107.9  grammes  is  dissolved  in  a  quantity  of 
nitric  acid  that  is  just  sufficient  to  take  it  all  into  solution,  then  the  same 
quantity  of  the  acid  will  just  dissolve  32.7  grammes  of  zinc;  and  the 
combining  weights  of  silver  and  zinc  in  corresponding  compounds  are 
always  in  the  ratio  of  107.9  to  32.7,  so  that  107.9  an(^  32<7  mav  be  called 
the  equivalent  quantities  in  chemical  combinations,  or  the  Equivalent 
Weights  of  these  metals.     Many  of  the  elements  are  not  confined  to  a 
single  relation  in  all  of  their  compounds,  but  possess  two  or  more  com- 
bining proportions  which  are  simple  multiples  of  each  other.     This  is 
true,  for  instance,  of  copper,  whose  usual  combining  proportions,  taken 
in  the  same  relation  as  those  given  above,  are  31.8  and   63.6.     The 
"equivalent  weights  "  or  "chemical  equivalents"  of  the  metals  are  im- 
portant factors  in  electrolytic  operations,  as  is  explained  in  Article  65. 

64.  Applications  of  Faraday's  Laws.  —  The  first  of  Faraday's  laws  of 
electrolysis  has  already  been  applied  to  the  operation  of  electric  bat- 


ELECTROLYSIS  5  5 

teries,1  and  a  battery  cell  is  in  fact  an  electrolytic  cell  in  which  the  process 
of  electrolysis  is  set  up  by  current  in  the  circuit  which  results  from  the 
chemical  activity  in  the  cell  itself.  The  chemical  changes  occurring 
through  the  consumption  of  zinc  in  the  primary  battery  cell  supply  suffi- 
cient energy  to  give  a  margin  for  useful  purposes  outside  of  the  cell ; 
somewhat  in  the  same  style,  for  instance,  as  fuel  is  consumed  at  the  boiler 
of  a  locomotive  for  the  purpose  of  converting  its  chemical  energy  into 
mechanical  power  sufficient  to  overcome  the  internal  frictional  and  other 
resistances  of  the  locomotive,  and  yet  leave  a  goodly  margin  of  power  for 
use  in  pulling  a  train. 

The  electrolytic  cell  which  is  intended  primarily  for  the  purpose  of 
decomposing  chemical  compounds,  ordinarily  requires,  on  the  other 
hand,  electrical  energy  to  be  applied  from  an  external  source  to  set  up 
the  chemical  decomposition.  In  fact,  there  are  comparatively  few  com- 
binations of  electrodes  and  electrolytes  that  will  give  out  sufficient  energy 
to  carry  on  desired  special  chemical  changes  in  themselves. 

Whether  the  power  which  is  used  for  forcing  the  current  through  the 
cell  is  obtained  from  the  cell  itself  or  from  an  external  source,  such  as  a 
dynamo  or  battery,  the  amount  of  chemical  action  caused  by  the  elec- 
trical current  in  a  given  time  is  proportional  to  the  current  flowing; 
and  the  power  expended  in  making  a  given  amount  of  chemical  change 
is,  after  transformation  losses  are  deducted,  equal  to  that  which  would 
be  given  out  or  absorbed  were  the  decomposed  elements  of  the  electro- 
lyte joined  together  again.  The  storage  battery  is  a  good  example  of 
this.  When  the  battery  is  being  charged,  power  is  required  to  send  the 
current  through  the  circuit  which  makes  the  chemical  changes  in 
the  battery.  When  the  battery  is  being  discharged,  the  constituents  of 
the  battery  return  to  their  original  state,  and  in  doing  so  give  out  an 
amount  of  power  equal  to  that  expended  in  charging  minus  the  inevitable 
losses  which  occur  in  the  circuit. 

Faraday  foresaw  that  this  must  be  true,  and  said  that,  "  If  the  electri- 
cal power  which  holds  the  elements  of  a  grain  of  water  in  combination  " 
(water  is  made  of  two  atoms  of  hydrogen  to  one  of  oxygen),  "or  which 
makes  a  grain  of  oxygen  and  hydrogen  in  the  right  proportions  unite  into 
water  when  they  are  made  to  combine,  could  be  thrown  into  the  condi- 

1  Article  53. 


56  ELECTRICITY   AND    MAGNETISM 

tion  of  a  current,  it  would  exactly  equal  the  current  required  for  the 
separation  of  that  grain  of  water  into  its  elements  again." 

65.  Electrochemical  Equivalents. — The  second  law  given  in  Article 
62  means  that  the  amount  of  chemical  change  that  takes  place,  when 
a  coulomb  of  electricity  is  passed  through  an  electrolytic  cell,  is  de- 
pendent upon  those  characteristics  of  the  elements  of  the  electrolyte 
which  may  be  called  their  equivalent  weights. 

If,  then,  we  have  weighed  the  amount  of  silver  that  is  separated  from 
a  silver  solution  by  the  passing  of  one  coulomb,  and  if  we  know  the 
equivalent  weights  of  the  various  metals,  we  can  calculate  the  amount 
of  any  other  metal  that  will  be  separated  under  similar  circumstances, 
by  merely  multiplying  the  weight  of  the  silver  separated  by  the  ratio  of 
the  equivalent  weight  of  the  other  metal  to  that  of  silver. 

The  weight  of  a  material  that  is  separated  from  an  electrolyte  by  one 
coulomb  of  electricity  is  called  its  Electrochemical  Equivalent.  The 
electrochemical  equivalents  of  a  few  of  the  chemical  "elements"  have 
been  determined  by  direct  measurements,  and  others  by  calculation 
from  their  relative  combining  proportions. 

The  following  is  a  table  of  the  values  of  the  usual  electrochemical 
equivalents  for  a  number  of  the  chemical  elements  :  — 


IONS 

Electrochemical  Equivalents 
in  Grammes  per  Coulomb 

Relative  Combining  Propor- 
tions or  Equivalent  Weights 

Aluminum        
Copper,  I    
Copper   II 

.0000936  equals  K 
.000659        "      K 
.000329        "       K 

times            9-°4 
63.6 
"      i  x  6^  6 

Gold       
Hydrogen   
Iron   II 

.000681         "       K 
.0000104      "       K 
.000290        "      K 

65.7 

1.008 

280 

Iron,  III      
Lead       
Nickel         

.000193         "       K 
.00107          "      K 
.000304        "      K 

-     -|  x  28.0 
103.4 

"                 2Q.4 

Nitrogen      
Oxygen        
Silver 

.0000485      "      K 
.0000829      "      K 
oo  1  1  1  8        "      lv 

4.68 

8.00 

'"                IO7  Q 

Tin,  II    
Tin,  IV  ... 

.000616        '•      K 
000308        "      K 

59-5 

"       o    X    SQ  S 

Zinc   

.000338        "       K 

32.7 

ELECTROLYSIS  57 

The  value  of  K  in  this  table  is  equal  to  the  electrochemical  equiva- 
lent of  any  one  of  the  elements  divided  by  the  corresponding  equivalent 
weight.  Its  numerical  value  is  .00001036.  The  values  of  the  equiva- 
lents for  silver  are  those  which  are  most  accurately  known. 

It  is  noticed  that  two  values  are  given  in  the  table  for  tin,  for  iron, 
and  for  copper.  This  is  because  each  of  these  metals  has  different 
chemical  equivalents  in  two  classes  of  compounds  of  common  occur- 
rence. The  second  value  of  the  electrochemical  equivalent  of  copper,  i.e. 
.000329,  is  the  value  which  usually  applies  when  copper  is  deposited  in 
an  electrolytic  cell.  The  table  may  be  reduced  to  common  measure 
for  service  in  electroplating,  etc.,1  by  the  knowledge  that  one  gramme 
(Metric  Measure)  is  equal  to  15.432  grains,  or,  approximately,  thirty- 
five  thousandths  of  an  ounce. 

PROBLEMS 

A.  How  many  ounces  of  silver  will  be  deposited  by  100,000  coulombs  of  elec- 
tricity ?     Ans.    3.91  oz.  (approx.). 

B.  How  many  ounces  of  copper  will  be  deposited  by  1,000,000  coulombs  of  elec- 
tricity from   a  copper  solution  having  an  electrochemical  equivalent   of  .000329  ? 
Ans.    11.5  oz.  (approx.). 

C.  How  many  ounces  of  aluminum  will  be  deposited  from  a  suitable  electrolyte  by 
100  amperes  flowing  for  one  hour?     Ans.    1.18  oz.  (approx.). 

D.  How  many  ounces  of  zinc  does  a  Daniell  cell  consume  in  generating  \  of  an 
ampere  of  current  for  3  weeks  continuously  ?     Ans.    5.35  oz.  (approx.). 

E.  How  many  ounces  of  acidulated  water  will  50  amperes  flowing  for  5  hours 
decompose?     Ans.    2.94  oz.  (approx.). 

66.  Electrolysis  of  Acidulated  Water  ;  Water  Voltameter.  —  Pure 
water  is  a  non-conductor;  but  if  a  little  sulphuric  acid  is  added,  it 
becomes  a  conductor,  and  it  may  then  be  decomposed  by  electrolysis. 
The  acid  is  first  decomposed,  and  that  in  turn  decomposes  the  water, 
but  the  effect  is  the  same  as  if  the  water  were  originally  decomposed. 
An  apparatus  such  as  that  illustrated  in  Figure  30  may  be  used  very 
nicely  for  showing  the  relation  between  the  electrochemical  equivalents 
of  hydrogen  and  oxygen.  The  tubes  A,  B,  and  C  are  filled  with  the 
acidulated  water  and  the  cocks  closed.  When  the  current  is  turned 
on,  it  flows  through  the  water  between  the  electrodes  EE,  and  the 

1  Chapter  XXII. 


ELECTRICITY   AND   MAGNETISM 


water  is  decomposed  so  that  oxygen  collects  in  A,  forcing  the  water  down ; 
and  hydrogen  collects  in  B,  also  forcing  the  water  down.  The  figure  shows 
that  there  is  almost  twice  the  volume  of  the  latter  as  of  the  former. 

To  show  why  the  liberated  gases  come  off  in 
these  proportional  volumes,  it  should  be  explained 
that  a  given  weight  of  hydrogen  occupies  about 
sixteen  times  as  much  bulk  or  volume  as  an  equal 
weight  of  oxygen  when  they  are  under  equal 
pressures.  Looking  at  the  table  of  the  preceding 
article,  it  is  seen  that  one  coulomb  of  electricity 
liberates  about  one-eighth  as  much  hydrogen  by 
weight  as  of  oxygen.  Therefore,  when  a  given 
weight  of  hydrogen  has  been  generated  in  the 
voltameter,  eight  times  as  great  a  weight  of  oxygen 
has  appeared  in  the  other  tube.  But,  because  of 
the  smaller  bulk  of  a  given  weight  of  oxygen  as 
compared  with  hydrogen,  the  amount  of  oxygen 
liberated  will  only  fill  about  one-half  as  much 
space  as  the  hydrogen.  The  distance  the  liquid 

is  pressed  down  in  the  hydrogen  tube  is  a  little  greater  than  twice  that 
in  the  other,  because  some  of  the  oxygen  is  dissolved  in  the  water. 

67.  Theory  of  Electrolysis.  —  The  reason  why  chemical  changes  take 
place  in  an  electrolytic  cell  has  not  as  yet  been  satisfactorily  explained, 
though  many  plausible  suggestions  have  been  made.  Probably  the 
one  most  usually  accepted  is  the  theory  of  Electrolytic  Dissociation. 
The  first  at  all  satisfactory  step  toward  this  theory  was  suggested  by 
Grotthuss  in  1805.  He  believed  that  the  metallic  parts  of  a  salt  were 
charged  with  a  positive  charge  of  electricity,  and  were  attracted  by  the 
cathode,  while  the  non-metallic  parts  were  attracted  in  the  opposite 
direction  towards  the  anode.  He  considered  that  the  current  flowing 
through  the  liquid  decomposed  it,  and  that  the  parts  were  then  caused 
to  move  to  the  electrodes  by  these  attractions.  Figure  31  represents 
his  conception,  which  is  intended  to  illustrate  his  idea  of  the  action 
of  the  current  upon  water.  The  ovals  in  the  figure  indicate  mole- 
cules, or  infinitely  small  masses  of  the  water.  The  squares  marked 
with  a  -f  sign  represent  the  hydrogen  atoms,  and  those  marked  with 


FIG.  30.  —  Water 
Voltameter. 


ELECTROLYSIS 


59 


FIG.  31.  —  Illustration  of  the  Ar- 
rangement of  Molecules  of 
Electrolyte  as  supposed  by 
Grotthuss. 


a  --  sign  represent  the  oxygen  atoms,  which  are  supposed  to  compose 
the  molecules  of  the  water,  and  which  are  held  together  by  the  attrac- 
tions of  their  electric  charges.  When  the 
electrodes  are  charged,  the  molecules  take 
the  positions  shown  in  the  figure.  The 
positive  charge  on  a  hydrogen  atom  next 
to  the  cathode  is  neutralized  by  the  nega- 
tive charge  on  the  plate.  This  atom  is 
thus  released  from  its  neighboring  oxy- 
gen atom  and  escapes.  The  freed  oxygen 
atom  then  combines  with  the  hydrogen 
in  the  next  molecule,  the  oxygen  of  that 
molecule  going  to  the  next,  and  so  on. 
The  last  oxygen  atom  is  freed,  as  was 
the  hydrogen,  by  contact  with  the  anode. 
New  molecules  have  now  formed  as  in- 
dicated by  the  brackets,  and  the  process  of  breaking  up  and  re-combin- 
ing is  continued. 

Since  a  negative  atomic  charge  has  been  given  to  the  anode,  and  a  posi- 
tive charge  to  the  cathode  during  the  process  described,  a  current  must 
flow  through  the  wire  connecting  the  terminals  of  the  cell,  as  shown  by  the 

arrows,  to  equalize  the  potentials,  and  the  elec- 
tricity continues  to  be  transferred  through 
the  cell  by  this  exchange  of  the  atoms. 

The  supposed  transfer  of  electricity  from 
electrode  to  electrode  by  the  atoms,  may  be 
likened,  by  analogy,  to  the  Electric  Chimes 
invented  by  Franklin  in  1752.  Figure  32 
represents  an  apparatus  somewhat  similar  to 
that  of  Franklin.  A  is  an  electrical  machine, 
BB*  are  two  bells  mounted  near  together 
but  insulated  from  each  other,  and  C  is  the 
tapper,  supported  by  a  silk  thread.  If  C  is 
touched  to  B,  it  is  given  a  positive  charge 
FIG.  32. -Electric  Chimes.  and  is  repelied  by  B  anci  attracted  by  B\ 
Analogy  to  the  Theory  J 

of  Electrolysis.  Flying  over  to  B ,  the  charge  of  C  is  reversed 


6o 


ELECTRICITY  AND    MAGNETISM 


by  contact  with  B1,  and  it  swings  to  B  again.  Thus  the  tapper  continues 
to  journey  back  and  forth  between  B  and  B'  ,  carrying  a  positive  charge 
'each  time  from  B  to  B'  and  returning  with  a  negative  charge  from  B'  to  B. 
The  machine  A  keeps  up  the  charges  on  the  bells  by  the  current  indi- 
cated by  the  arrows,  just  as  the  electric  battery,  B,  keeps  up  the  current 
through  the  electrolytic  cell  of  Figures  31  and  33.  The  bells  are  in 
metallic  connection  with  the  machine  by  means  of  wires. 

In  the  analogy  with  Grotthuss'  explanation  of  the  action  in  the  elec- 
trolytic cell,  we  must  think  of  the  bell  tappers  as  infinite  in  number,  and 
charged  from  an  external  source  ;  while  as  soon  as  they  tap  upon  the 
electrode  (the  bell)  they  escape.  The  continual  transfer  of  electricity 
from  electrode  to  electrode  is,  however,  of  a  similar  character  to  that 
performed  by  the  "  chimes." 

Clausius,  about  1857,  found  that  this  theory  would  not  explain  all  the 
phenomena  encountered  in  electrolysis.  He  considered  that  Grotthuss' 
explanation  of  the  transfer  of  electricity  was  satisfactory,  but  asserted 
that  a  sufficient  cause  did  not  exist  to  incite  the  breaking  up  of  the 
molecules  as  described.  He  therefore  presented  another  hypothesis,  to 
the  effect  that  when  a  salt  is  dissolved  or  diluted  in  water  some  of  the 
molecules  are  dissociated,  or  broken  up  into  their  component  atoms, 

by  the  process  of  solution.  If  common 
^  (which  is  a  compound  of  metallic  so- 
dium  and  chlorine)  is  dissolved  in  water, 
some  of  the  molecules  of  the  dissolved  salt 
are  broken-  up  into  their  atoms,  Clausius 
asserted,  and  the  solution  therefore  con- 
tains some  complete  molecules  of  salt  and 
also  some  free  atoms  of  sodium  and  of 
chlorine.  Figure  33  illustrates  the  case, 
where  the  ovals  represent  the  complete 
molecules  and  the  squares  the  free  ele- 
ments. The  positive  squares  in  this  case 
represent  sodium,  and  the  negative  ones 
represent  chlorine.  Clausius  held  that  the 

electrolytic  action  now  goes  on  by  the  free  atoms  discharging  at  the 
electrodes  as  assumed  in  Grotthuss'  theory. 


01°!  ^T^^K 
•  I  •  \ 

_J 
£ 


FlG.  33.  —  Illustration  of  the  Ar- 
rangement of  Molecules  and 
Atoms  in  a  Solution,  as  sup- 
posed by  Clausius. 


ELECTROLYSIS  6 1 

This  theory,  which  has  strong  experimental  evidence  for  its  support 
and  has  been  greatly  extended  by  various  noted  scientists  working 
within  the  past  two  decades,  has  done  much  to  aid  in  the  develop- 
ment of  electrochemistry  by  affording  experimenters  a  reasonably 
satisfactory  basis  from  which  to  examine  the  character  of  their  work ; 
but  recent  investigations  have  thrown  doubt  upon  its  truth.  The  reader 
must  remember  that  it  is  only  a  theory  to  be  used  for  the  purpose  of 
gaining  a  graphic  idea  of  the  processes  of  electrolysis,  and  may  there- 
fore be  looked  upon  as  akin  to  an  analogy  to  be  used,  like  the  hydraulic 
analogy  to  the  flow  of  electric  current,  only  for  the  purpose  of  clear 
illustration. 

QUESTIONS 

1.  What  two  kinds  of  electrical  conduction  are  there? 

2.  What  effect  has  an  electric  current  upon  solutions  of  salts  and  acids? 

3.  What  is  an  electrolyte? 

4.  What  is  electrolysis? 

5.  What  is  an  electrode? 

6.  What  are  the  two  electrodes  of  an  electrolytic  cell  called  ? 

7.  What  are  ions? 

8.  How  may  the  salt  of  a  metal  be  obtained  ? 

9.  Describe  the  action  when  a  current  is  sent  through  an  electrolytic  cell  which 
has  copper  plates  dipped  in  a  solution  of  copper  sulphate. 

10.  To  which  electrode  do  the  metal  ions  go? 

11.  What  happens,  in  a  cell  having  platinum  electrodes  and  a  solution  of  copper 
sulphate,  when  a  current  passes  between  the  electrodes? 

12.  When  did  Fara-Jay  make  his  investigations  on  the  decomposition  of  electro- 
lytes ? 

13.  What  is  Faraday's  first  law  of  electrolysis? 

14.  What  is  the  chemical  action  in  an  electrolytic  cell  proportional  to? 

15.  How  many  more  grammes  of  material  will  be  decomposed  by  25  coulombs 
than  by  5  coulombs? 

1 6.  What  is  Faraday's  second  law? 

17.  What  does  Faraday's  second  law  mean? 

1 8.  What  is  meant  by  "  chemical  equivalent  "  ? 

19.  Compare  a  primary  battery  cell  to  an  electrolytic  cell. 

20.  How  much  energy  is  required  to  produce  the  chemical  action  which  occurs 
in  an  electrolytic  cell? 

21.  What  is  the  electrochemical  equivalent  of  a  substance? 

22.  A  gramme  is  what  part  of  an  ounce? 


62  ELECTRICITY   AND    MAGNETISM 

23.  Describe  how  oxygen  and  hydrogen  can  be  separated  from  acidulated  water. 

24.  Why  is  the  bulk  of  hydrogen  liberated  by  the  decomposing  of  water  twice  as 
great  as  the  bulk  of  oxygen  liberated  ? 

25.  In  decomposing  water,  how  much  more  weight  of  oxygen  is  liberated  than  of 
hydrogen?     Why? 

26.  What  is  meant  by  the  theory  of  electrolytic  dissociation? 

27.  What   did  Grotthuss    do    in    the    development   of  the   dissociation  theory  ? 
When? 

28.  Illustrate  Grotthuss'  conception  of  the  action  in  a  cell. 

29.  Give  a  mechanical  or  electrical  analogy  to  the  action  in  a  cell,  as  explained  by 
Grotthuss  (electric  chimes). 

30.  What  is  Clausius'  hypothesis  regarding  electrolytic  action?      When  did  he 
advance  it  ? 

31.  Has  the  electrolytic  dissociation  theory  been  proved? 


CHAPTER   VI 

THE  NATURE  AND   PROPERTIES   OF   MAGNETISM 

68.  Historical  Facts  Concerning   Magnetism.  —  The  true  nature  of 
magnetism  seems  to  be  very  closely  connected  with  that  of  electricity, 
and  it  will  probably  not  be  exactly  known  till  the  exact  nature  of  elecr 
tricity  is  determined.   The  word  Magnet  probably  comes  from  the  Greek 
word  for  the  country  of  Magnesia,  which  is  a  small  division  of  Ancient 
Greece,  where  a  deposit  of  magnetic  iron  ore  or  Lodestones  (also  called 
loadstones)  was  known  to  the  Greeks. 

Some  of  the  properties  of  magnets  were  known  long  before  the 
Christian  era.  It  is  said  that  the  Chinese  used  a  device  similar  to  the 
compass  to  guide  their  way  across  the  plains  of  Tartary  many  centuries 
before  the  birth  of  Christ,  but  this  is  not  probable.  In  Europe  the  use 
of  the  compass  did  not  become  general  until  the  thirteenth  century  of 
the  Christian  era.  The  attractive  power  which  magnets  have  for  iron  is 
mentioned  by  many  early  writers  :  Plato,  Euripides,  and  Thales  (the 
Greek  philosopher  referred  to  in  Article  i),  all  speak  of  the  lodestone 
or  magnet.  Dr.  Gilbert,  who  laid  the  foundation  for  our  words  "  elec- 
trical "  and  "electricity,"  made  a  great  many  experiments  with  magnets 
and  magnetic  materials,  and  was  the  first  to  recognize  that  the  earth  is  a 
great  magnet. 

69.  Artificial  and  Natural  Magnets.  —  Lumps  of  iron  ore  composed 
of  a  certain  oxide  of  iron  which  is  called  Magnetite  or  magnetic  ore, 
when  in  a  pure  form,  sometimes  have  the  peculiar  property  of  attracting 
pieces  of  iron,  and  they  are  then  called  lodestones.    The  property  held 
by  the  lodestone  is  called  Magnetism,  and  the  body  having  the  property 
of  magnetism  is  called  a  Magnet. 

The  action  of  magnets  led  some  of  the  earlier  experimenters  to  look 
upon  magnetism  as  due  to  a  magnetic  fluid,  but  this  idea  has  been 
proved  to  be  wrong.  It  is  found  that  pieces  of  steel  which  touch  a 
lodestone,  or  other  magnet,  become  magnets  without  any  loss  of  the 

63 


64 


ELECTRICITY   AND    MAGNETISM 


magnetic  virtue  from  the  original  magnet,  —  which  would  not  be  possible 
if  magnetism  were  an  ordinary  fluid.  Magnets  made  thus  by  touching 
are  sometimes  called  Artificial  Magnets,  and  lodestones  are  called 
Natural  Magnets.  When  pieces  of  soft  iron  touch  a  magnet  they  also 
become  magnets,  or  are  Magnetized,  while  in  contact  with  the  magnet ; 
but  when  separated  from  it  the  magnetism  of  the  soft  iron  disappears. 
This  is  called  Temporary  Magnetism,  while  the  magnetism  of  hard  steel 
which  remains  permanently  is  called  Permanent  Magnetism. 

70.  North  and  South  Poles  and  the  Magnetic  Needle.  —  When  a 
magnet  is  suspended  on  a  pivot  or  a  thread,  it  sets  itself  in  a  direction  so 
as  to  point  nearly  north  and  south ;  and  in  our 
country,  if  it  is  pivoted  at  the  centre,  the  north 
end  dips  down  as  though  it  were  heavier  than 
the  south  end.  A  small  elongated  magnet  thus 
suspended  is  called  a  Magnetic  Needle  (Fig.  34). 
If  a  magnetic  needle  is  turned  from  the  direc- 
tion which  it  naturally  takes  when  free  to  swing 
horizontally  on  its  pivot,  it  will  at  once  return, 
swinging  to  and  fro  until  it  settles  down  in  its 
original  position. 

The  pole  of  a  suspended  needle  which  points  to  the  north  is  called 
the  North  Pole,  and  the  other  pole  is  called  the  South  Pole.  This  ten- 
dency of  a  magnetic  needle  to  set  itself  north  and 
south  is  the  foundation  of  the  compass,  which  essen- 
tially consists  of  a  magnetic  needle  mounted  over  a 
dial.  It  is  usual  in  compasses  to  counterbalance 
the  needle,  or  pivot  it  so  that  it  will  hang  horizon- 
tally, but  Dip  Needles  are  sometimes  constructed 
by  mounting  magnetic  needles  on  horizontal  pivots 
(Fig.  35).  When  a  dip  needle  is  turned  north  and 
south,  in  our  country  its  north  pole  turns  to  a 
certain  degree  down  toward  the  earth,  as  already 
explained. 

The  north  pole  of  a  magnet  is  often  called  the 
Positive  or  Plus  (  +  )  Pole,  and  the  south  pole  is  often  called  the  Nega- 
tive or  Minus  (  — )  Pole.     Since  the  north  or  positive  pole  turns  toward 


FiG.  34.  —  Magnetic 
Needle. 


FIG.  35.  —  Dip  Needle. 


THE  NATURE  AND    PROPERTIES   OF   MAGNETISM  65 

the  north,  it  is  sometimes  called  the  North-seeking  Pole,  and  the  south 
or  negative  pole  is  sometimes  called  the  South-seeking  Pole. 

71.  Variation  of  Compass  and  Dip  Needles.  —  It  was  originally  sup- 
posed that  a  magnetic  needle  always  pointed  toward  the  same  point  on 
the  earth,  that  is,  that  it  always  pointed  in  a  direction  which  was  fixed 
with  respect  to  the  true  north.     But  Columbus  made  a  discovery  on  his 
first  voyage  to  America  which  upset  the  old  ideas.     His  sailors  dis- 
covered, to  their  great  excitement  and  fear,  that  the  direction  of  their 
ships'  compass  needles  gradually  changed  as  the  ships  sailed  along.    The 
needles  kept  pointing  more  and  more  away  from  the  direction  of  the 
North  Star,  and  the  sailors  became  greatly  alarmed  on  account  of  this 
most  unexpected  state  of  affairs.     Had   they  known  what  we  now  know, 
—  that  the  earth  is,  in  effect,  a  great  magnet,  and  that  its  magnetic  poles 
do  not  exactly  correspond  with  the  geographical  poles,  —  they  would 
have  had  no  cause  for  alarm.     As  it  was,  their  discovery  of  a  difference 
at  different  parts  of  the  earth's  surface  in  the  variation  of  the  magnetic 
needle,  from  the  direction  of  the  true  north,  made  an  important  contri- 
bution to  our  present  knowledge  of  the  earth's  magnetic  effects. 

The  dip  of  the  needle  also  varies  from  place  to  place.  At  the  region 
on  the  earth  called  the  north  magnetic  pole,  the  needle  stands  with  its 
north  pole  down  and  its  direction  straight  to  the  earth,  or  the  dip  is  90°. 
As  the  needle  is  carried  farther  and  farther  away  from  the  earth's  pole  the 
dip  becomes  less,  until  the  earth's  magnetic  equator  is  approached, 
where  the  needle  stands  in  a  horizontal  position,  or  the  dip  is  zero.  If 
the  needle  is  now  carried  farther  south  it  comes  more  directly  under  the 
influence  of  the  earth's  south  magnetic  pole,  and  the  south  pole  of  the 
needle  dips  down. 

72.  Magnetic  Attraction  and  Repulsion.  —  If  a  pole  of  a  magnet  is 
brought  near  a  magnetic  needle  it  is  found  to  attract  one  pole  of  the 
needle  and  repel  the  other  pole.    The  north  pole  of  the  magnet  may  be 
determined  by  noting  the  way  it  stands  when  suspended  by  a  thread, 
and  it  will  then  be  found  that  the  north  pole  of  the  magnet  always  repels 
the  north  pole  of  the  needle  and  attracts  the  south  pole    of  the  needle. 
The  south  pole  of  the  magnet  acts  in  an  exactly  opposite  manner. 

This  action  shows  that  there  are  tivo  kinds  of  magnetic  poles,  and 
that  poles  of  the  same  kind  repel  each  other  and  poles  of  opposite  kinds 


66' 


ELECTRICITY   AND   MAGNETISM 


attract  each  other.  This  is  quite  similar  to  the  law  of  attractions  and 
repulsions  of  electric  charges  given  in  Article  4.  Now  that  we 
know  this  law  of  magnetic  attraction  and  repulsion,  we  understand  why 
the  attractions  of. the  great  earth  magnet  cause  magnetic  needles  to  take 
certain  positions  as  described  in  the  previous  paragraphs. 

73.  Induced  Magnetism.  —  If  the  experiment  with  a  magnetic  needle, 
described  in  the  paragraph  above,  is  repeated,  but  a  bar  of  soft  iron  is 
used  in  place  of  the  magnet,  it  is  found  that  either  end  of  the  iron  bar 
attracts  either  pole  of  the  needle.  If  the  iron  bar  is  laid  with  one  end 
near  the  pole  of  a  magnet,  it  may  be  shown  to  be  magnetized  by  mov- 
ing a  magnetic  needle  around  it.  The  needle  will  show  by  its  action  that 
the  end  of  the  iron  bar  which  is  near  the  magnet  pole  has  become  a  pole 
of  sign  opposite  to  that  of  the  magnet  pole,  and  the  farther  end  of  the 
bar  has  become  a  pole  of  the  same  sign  as  that  of  the  magnet  pole.  The 
bar  is  said  to  be  Magnetized  by  Induction. 

The  magnetism  in  the  iron  bar  becomes  stronger  as  it  is  brought 
closer  to  the  magnet  pole.     The  magnetism  induced  in  a  bar  of  iron 
may  induce  magnetism  in  another  piece,  and  this  in  another  piece,  and 
so  on ;  and  thus  a  magnet  may  be  made  to  support  a  string  of  several 
nails  end  to  end,  each  of  which   has  become  a 
magnet  by  induction  (Fig.  36).     But  the  mag- 
netism in  each  successive  piece  is  weaker  than  in 
the  preceding  piece. 

We  are  now  in  a  position  to  see  why  a  magnet 
attracts  a  piece  of  iron,  and  the  cause  for  the 
effect  of  the  iron  bar  on  the  magnetic  needle. 
When  a  steel  magnet  pole  is  brought  near  to  a 
piece  of  iron,  the  iron  is  magnetized  by  induc- 

FIG.  36.-String  of  Nails    tion>    The  Positive  and  negative  poles  induced  in 
magnetized  by  indue-    the  iron  are  of  equal  magnitude  or  strength.    One 

V°H  ?nd  *r?ended    of  tlie  induced  poles  is  attracted  and  the  other  is 
End  to   End  from  a 

Magnet.  repelled  by  the  steel  magnet  pole,  but  that  which 

is  attracted  is  nearest  the  original  magnet  pole, 

and  the  force  of  attraction  is  therefore  greater  than  the  force  of  repul- 
sion. The  effect  of  a  bar  of  iron  on  a  magnetic  needle  is  caused  in  the 
same  way  by  the  magnetism  induced  in  the  bar  by  the  poles  of  the  needle. 


THE  NATURE  AND   PROPERTIES  OF   MAGNETISM  6/ 

74.  Every  Magnetic  Body  contains  Two  Poles  of  Opposite  Signs.  — 

i.  For  every  pole  induced  in  a  piece  of  iron  or  steel,  another 
equal  pole  is  produced.  For  instance,  if  the  north  pole  of  a  magnet 
is  touched  to  one  end  of  a  bar  of  iron,  a  south  pole  is  induced  in  that 
end  and  an  equal  north  pole  in  the  other  end.  2.  If  the  two  ends  of 
the  iron  bar  are  touched  at  once  by  the  north  poles  of  two  equal  mag- 
nets, south  poles  are  induced  in  both  ends  of  the  bar.  In  this  case  an 
examination  of  the  bar  with  a  magnetic  needle  shows  that  a  north  pole, 
which  is  equivalent  to  two  poles,  is  produced  near  the  centre  of  the  bar, 
and  this  pole  in  the  middle  is  called  a  Consequent  Pole.  3.  Again,  if 
a  magnet  is  broken,  it  is  found  that  each  piece  has  two  equal  and 
opposite  poles. 

We  are  therefore  justified  in  saying  that  for  every  magnet  pole  that 
exists,  there  also  exists  in  the  same  magnetic  body  an  equal  and  opposite 
pole.  This  is  quite  similar  to  the  existence  along  with  every  electric 
charge  of  an  equal  and  opposite  charge,  as  is  explained  in  Article  5. 

Magnetic  force  acts  through  a  vacuum  and  through  all  materials  ex- 
cept those  in  which  magnetism  may  be  induced. 

75.  Magnetic  and  Non-magnetic  Materials.  — Material  in  which  mag- 
netism may  be  induced,  and  which  is  therefore  attracted  by  a  magnet, 
is  called  Magnetic  Material.     Iron,  in  its  various  forms  (such  as  wrought 
iron,  cast  iron,  and  steel),  is  the  most  strongly  magnetic  material  known. 
There  are  only  a  few  other  materials  that  are  known  to  be  magnetic.    Of 
these  the  metals  called  nickel  and  cobalt  are  the  commonest.  Manganese, 
platinum,  some  of  the  salts  of  magnetic  metals  with  their  solutions,  and 
oxygen  are  more  or  less  magnetic,  though  usually  to  a  very  slight  degree. 
All  materials  which  are  not  quite  strongly  magnetic  are  usually  spoken  of 
as  Non-magnetic,  since  they  are  nearly  neutral  as  regards  magnetism. 
Magnetic  materials  are  sometimes  called  Paramagnetic,  and  non-mag- 
netic materials  are  sometimes  called  Diamagnetic,  but  these  terms  have 
also  additional  scientific  meanings  which  heed  not  be  discussed  here. 

Whenever  magnetic  material  is  needed  in  the  useful  arts,  iron  in  its 
various  forms,  such  as  cast  iron,  wrought  iron,  and  steel,  is  almost  exclu- 
sively used  on  account  of  its  great  magnetic  qualities.  In  fact,  other 
materials,  except  nickel  and  cobalt,  may  be  considered  to  be  practically 
neutral  when  compared  with  iron. 


68 


ELECTRICITY  AND   MAGNETISM 


FIG.  37.  —  Bar 
Magnet. 


76.  Forms  of  Magnets  and  Methods  of  making  Them.  —  Magnets  may 
be  made  of  almost  any  form.     Those  made  of  straight  pieces  of  steel  are 

called  Bar  Magnets  (Fig.  37),  and  those  with  poles  bent 
nearly  together,  Horseshoe  Magnets  (Fig.  38).  A  magnet 
made  in  the  form  of  a  thin  disk,  which  is  magnetized  so 
that  its  two  flat  surfaces  are  the  poles,  is  called  a  Magnetic 
Shell.  The  latter  is  not  a  very  useful  form  except  for 
theoretical  scientific  discussions.  Electromagnets,  which 
will  be  treated  later,  may  be  found  in  a  great  variety  of 
forms,  depending  upon  the  duty  which  they  are  intended 
to  perform.  In  making  permanent  magnets  of  large  cross 
section,  it  is  common  to  lay  several  thin  magnets  one  upon 
the  other.  This,  as  will  be  seen  later,  makes  stronger 
poles.  A  good  permanent  magnet  so  made  should  be  able  to  lift 
twenty  times  its  own  weight. 

A  steel  bar  may  be  magnetized  by  rubbing  one  end 
with  the  north  pole  of  a  magnet,  and  the  other  end  with 
the  south  pole ;  or  two  magnets  may  be  used,  stroking 
the  bar  with  both  at  the  same  time,  from  the  middle 
outward,  using  opposite  poles.  Much  stronger  results 
may  be  obtained,  however,  by  placing  the  steel  bar 
against  a  strong  electromagnet;  but  the  ends  of  the 
bar  should  be  joined  by  a  piece  of  soft  iron  or  the 
electro-magnet  discharged  before  the  steel  bar  is  re- 
moved, lest  the  induced  magnetism  be  partially  de- 
stroyed in  the  process  of  drawing  the  bar  away.  The  FlG  38  _  Horse. 
bar  may  also  be  magnetized  by  placing  it  within  a  coil  of  shoe  Magnet. 
wire  through  which  a  current  of  electricity  is  flowing. 

77.  Demagnetization  and  Effect  of  Heat.  —  Any  jarring  of  a  magnet 
will  tend  to  cause  its  magnetism  to  disappear,  or  to  Demagnetize  it.     A 
few  sharp  strokes  of  a  hammer  or  the  scratch  of  a  file  may  cause  the 
greater  part  of  the  magnetism  to  disappear  ;  also  if  the  magnet  is  heated 
to  a  temperature  about  red  heat,  it  becomes  demagnetized,  and  the  iron 
at  the  same  time  loses  its  magnetic  quality  and  does  not  regain  it  until 
it  cools  to  a  lower  temperature.     Cooling  a  steel  magnet  seems  to  in- 
crease its  magnetism  slightly.     The  effect  which  is  caused  upon  the  mag- 


THE  NATURE  AND   PROPERTIES  OF   MAGNETISM  69 

netic  quality  of  nickel  and  cobalt  by  heating  them  is  similar  to  the  effect 
on  iron,  but  these  metals  lose  their  magnetic  quality  at  lower  tempera- 
tures. The  reason  for  this  curious  effect  is  entirely  unknown,  but  it  is 
supposed  to  come  about  through  some  action  on  the  molecules  of  the 
material.  Some  very  curious  results  may  be  produced  in  the  magnetic 
qualities  of  certain  grades  of  nickel  steel  and  other  steel  alloys  by  heat- 
ing and  cooling  them. 

As  a  magnet  loses  its  magnetism  so  readily  through  handling,  the 
horseshoe  form  is  usually  furnished  with  a  keeper.  The  keeper  is  a 
piece  of  soft  iron  which  may  be  placed  across  the  poles  of  the  magnet. 
This  makes  a  complete  magnetic  circuit  for  the  magnetism,  and  tends  to 
prevent  its  destruction. 

78.  Coercive  Force.  —  It   is    found    that   some   materials   are   more 
readily  magnetized  and  demagnetized  than  others.     It  is  well  known,  for 
instance,  that  soft  iron  is  very  readily  magnetized,  but  loses  almost  all 
of  its  magnetism  if  it  is  slightly  jarred  after  the  external  magnetizing 
force  is  withdrawn. 

Hard  steel  is  usually  more  difficult  to  magnetize,  but  it  retains  its 
magnetism  quite  strongly.  Generally  speaking,  the  harder  the  steel  the 
more  difficult  it  is  to  magnetize,  and  the  more  strongly  it  retains  its 
magnetism.  We  are  driven,  then,  to  the  belief  that  there  is  some  force 
that  opposes  the  magnetization  of  magnetic  materials  and  also  opposes 
their  demagnetization.  This  force,  which  is  supposed  to  hinder  changes 
of  magnetic  strength  or  condition,  is  called  Coercive  Force,  and  it  is 
much  stronger  in  hard  steel  than  in  soft  iron.  The  effect  of  the  coer- 
cive force  is  counteracted  to  some  extent  by  anything  that  is  likely  to 
make  the  molecules  vibrate,  such  as  rough  handling,  heating,  etc.  As 
has  already  been  said,  heating  to  a  red  heat  will  cause  a  magnet  to  lose 
all  of  its  magnetism,  and  a  magnet  which  is  dropped  on  the  floor  a  few 
times  will  lose  much  of  its  magnetism. 

79.  Saturation.  — When  a  magnet  is  magnetized  as  strongly  as  pos- 
sible, it  is  said  to  be  Saturated,  and  when  the   magnetism  has  reached 
this  point  it  will  generally  grow  weaker  for  a  certain  time  after  magnet- 
izing, if  the  magnetizing  force  is  removed  and  the  magnet  is  left  alone, 
till  the  magnetism  finally  becomes  permanent  in  strength.     Such  perma- 
nent magnets,  which  have  lost  the  temporary  magnetism  due  to  satura- 


/O  ELECTRICITY  AND   MAGNETISM 

tion,  are  very  useful  parts  of  many  electrical  instruments  where  a 
constant  magnetic  effect  is  important ;  so  that  Aged  magnets,  as  they  are 
called,  are  regularly  manufactured.  They  may  be  artificially  aged  by 
immersing  in  steam  for  a  considerable  time. 

80.  Distribution  of  Magnetism.  —  The  poles  of  a  long  bar  magnet  are 
not  entirely  gathered  at  the  ends,  but  extend  some  distance  along  the 


fc/Ax-VT  >,  '/  f  f\  iy^\X*Zr^^^^  —         t^>  /If  f/If  I  m\\l\\\  v    •/   |V» 


FIG.  39.  —  Picture  of  Iron  Filings  showing  the  Distribution  of  Magnetism  about  a  Bar 

Magnet. 

sides  as  indicated  by  the  iron  filings  in  Figure  39.  If  the  bar  is  very 
long  and  not  made  of  homogeneous  material,  several  consequent  poles 
may  appear  (see  Figure  40),  much  as  would  be  the  case  if  the  bar  were 
made  up  of  several  magnets  with  their  several  like  poles  together.  If 


FIG.  40.  —  Picture  of  Iron  Filings  showing  the  Distribution  of  Magnetism  about  a  Bar 
Magnet  with  Two  Consequent  Poles. 

a  very  large  block  or  sheet  of  steel  is  touched  at  different  places  by  a 
magnet  pole,  poles  will  appear  at  the  points  touched.  If  a  horseshoe 
magnet  is  closed  at  the  poles  by  a  piece  of  iron  (called  a  keeper)  very 
little  of  the  magnetism  will  be  evident,  as  will  be  explained  later.  In 


THE  NATURE  AND   PROPERTIES   OF  MAGNETISM  71 

magnetizing  hard  steel  the  surface  only  is  likely  to  be  much  affected ; 
therefore,  in  making  a  permanent  magnet  having  a  large  cross  section,  it 
is  advisable,  as  was  suggested  above,  to  fasten  together  a  number  of  thin 
magnets.  Such  a  magnet  is  said  to  be  Laminated. 


QUESTIONS 

1.  What  is  the  probable  derivation  of  the  word  magnet? 

2.  In  what  century  did  the  use  of  the  magnetic  needle  become  usual? 

3.  What  is  a  lodestone? 

4.  What  is  magnetism? 

5.  What  is  a  magnet? 

6.  What  happens  to  a  piece  of  steel  when  it  is  touched  by  a  piece  of  lodestone? 

7.  What  happens  to  a  piece  of  soft  iron  when  it  is  touched  by  a  piece  of  lode- 
stone? 

8.  What  is  temporary  magnetism? 

9.  What  is  permanent  magnetism? 

10.  What  is  a  magnetic  needle? 

11.  W7hat  position  does  a  magnetic  needle  take  in  America  when  allowed  to 
swing  free  in  all  directions? 

12.  What  are  the  ends  of  the  magnetic  needle  respectively  called? 

13.  WThat  is  a  compass?     A  dip  needle? 

14.  What  discovery  did  Columbus  make  with  reference  to  the  magnetic  needle? 

15.  If  the  north  pole  of  a  magnet  is  brought  near  a  magnetic  needle,  what  results? 

1 6.  If  two  south  poles  are  brought  near  together,  how  will  they  act? 

1 7.  State  the  law  which  magnet  poles  follow  in  their  action  upon  one  another. 

1 8.  What  is  magnetic  induction? 

19.  If  a  piece  of  soft  iron  is  brought  near  the  positive  pole  of  a  magnet,  what  kind 
of  a  pole  is  induced  in  the  iron  nearest  the  magnet  pole? 

20.  Why  will  either  pole  of  a  magnetic  needle  be  attracted  to  a  soft  piece  of  iron? 

21.  Can  a  single  pole  exist  alone? 

22.  WThat  is  a  consequent  pole? 

.   23.  Need  the  poles  of  a  magnet  be  only  at  the  ends  of  a  steel  bar? 

24.  What  materials  will  magnetic  forces  act  through? 

25.  Wrhat  are  the  known  magnetic  materials? 

26.  What  is  the  meaning  of  paramagnetic? 

27.  What  is  the  meaning  of  diamagnetic? 

28.  What  are  bar  magnets?     Horseshoe  magnets? 

29.  What  is  a  magnetic  shell? 

30.  Name  a  number  of  ways  by  which  a  steel  bar  may  be  magnetized. 

31.  About  how  much  should  a  well  made  steel  magnet  lift? 


72  ELECTRICITY  AND    MAGNETISM 

32.  If  a  magnet  be  struck  by  a  hammer,  what  is  likely  to  happen? 

33.  What  happens  if  a  magnet  is  heated  red-hot  ? 

34.  Will  a  magnetic  needle  be  attracted  by  a  piece  of  red-hot  iron? 

35.  What  is  a  keeper?     What  is  it  for? 

36.  What  is  coercive  force? 

37.  Which  is  the  harder  to  magnetize,  soft  iron  or  steel? 

38.  Which  has  the  greater  coercive  force,  soft  iron  or  steel? 

81.  What  is  Magnetism?  —  We  do  not  know  what  magnetism  is,  but 
we  know  a  great  deal  about  its  effects  (all  of  which  has  been  learned  by 
experiments  and  experience),  and  we  have  theories  about  its  real 
nature.  Scientific  theories,  it  must  be  remembered,  are  nothing  more 
than  shrewd  guesses  at  the  secrets  of  nature,  —  the  guesses  being 
based  on  the  foundation  of  all  that  we  know,  —  and  these  theories  are 
being  continually  altered  and  improved  as  more  facts  are  learned  by 
experience.  The  earliest  theories  which  offered  fairly  complete  expla- 
nations of  the  various  phenomena  of  magnetism  were  outlined  by  Coulomb 
(after  whom  the  unit  quantity  of  electricity,  the  coulomb,  was  named), 
about  1785,  and  by  Poisson  (a  great  mathematician  of  France),  about 
1821.  They  were  followed  by  a  host  of  others  which  return  more  or 
less  satisfactory  results  when  put  to  the  test  of  experiment.  Since,  into 
however  many  pieces  a  magnet  may  be  broken,  each  piece  shows  a  north 
and  south  pole,  it  has  long  been  considered  that  magnetism  is  molecular 
in  nature ;  that  is,  that  the  smallest  particles  or  molecules,  into  which 
the  material  can  be  theoretically  divided,  are  little  magnets,  each  of 
which  has  its  own  north  and  south  poles.  So  that  the  theories  in  re- 
gard to  the  nature  of  magnetism,  that  have  been  proposed  from  time  to 
time,  are  all  based  upon  this  idea  of  "  polarization  "  in  the  molecules  of 
magnetic  material. 

The  theory  of  Coulomb,  which  was  used  and  extended  by  Poisson, 
regarded  all  molecules  as  containing  equal  parts  of  two  magnetic  fluids, 
one  called  "Austral,"  or  southern,  and  the  other  "Boreal,"  or  northern, 
which  were  supposed  to  be  equally  mixed  under  ordinary  conditions. 
But  when  the  molecules  were  brought  within  the  influence  of  a  magnet, 
the  fluids  were  supposed  to  separate  and  occupy  opposite  halves  of  the 
molecules  and  so  produce  magnet  poles  at  each  end  of  the  piece  of 
influenced  magnetic  material.  This  theory  had  many  faults,  and  was 


THE  NATURE  AND   PROPERTIES  OF   MAGNETISM  73 

soon  replaced  by  one  proposed  by  Ampere  (another  great  Frenchman) 
about  1830.  In  Ampere's  theory,  each  molecule  of  magnetic  material 
is  supposed  to  be  magnetized  by  an  electric  current  which  flows  around 
it.  When  a  bar  of  magnetic  material  is  not  magnetized,  the  molecules 
are  supposed  to  be  arranged  haphazard,  but  in  such  order  as  to  neutral- 
ize each  other  in  external  magnetic  effect.  When  the  material  is  placed 
near  a  magnet  pole,  the  molecules  are  supposed  to  be  swung  around  by 
its  attraction  or  repulsion  until  their  axes  are  approximately  parallel  and 
their  like  poles  all  pointing  one  way.  Experimental  facts  indicate  that 
it  is  doubtful  whether  such  an  electric  current  can  circulate  about  the 
molecules,  and  there  is  no  good  reason  to  believe  that  it  does,  and  so 
this  theory  may  also  be  discarded. 

The  theory  that  is  now  generally  accepted,  and  which  seems  to  more 
nearly  apply  to  the  true  condition  of  the  molecules,  was  first  -advanced 
by  Weber,  about  1852,  and  was  used  by  Maxwell  in  his  profound  mathe- 
matical investigations.  In  this  theory  the  molecules  of  magnetic  matter 
are  supposed  to  be  magnets  by  nature,  that  is,  they  are  supposed  to  have 
natural  magnetic  poles  which  are  just  like  those  of  ordinary  magnets,  but 
of  course  they  are  very  small  ones ;  and  we  therefore  say  that  magnetic 
attractions  between  bits  of  magnetic  material,  whether  they  are  large  or 
small,  are  just  as  natural  as  the  attractions  between  bodies  which  we  call 
gravitation.  Now,  when  magnetic  material  is  unmagnetized  it  is  sup- 
posed that  the  molecules  are  arranged  in  a  haphazard  manner,  or  in 
haphazard  groups,  so  that  they  neutralize  each  other's  external  mag- 
netic effects ;  but  when  the  material  is  subjected  to  the  influence  of 
magnetic  force,  the  molecular  magnets  are  all  attracted  around  so  that 
their  poles  point  more  or  less  in 
the  same  direction.  In  Figure  . .  3a»^S33 czSj3Sj»Sa5S  ^ 

N  ^TS*  m  Ei-**  cm  QfT*  SJ^  ^r^a  -^  I:B> 

41  the  small  blocks  may  be  taken       [?i£^^3  McSIycSLSssfaiS 

to  roughly  represent  the  mag-  FIG.  41.  —  Illustration  of  Arrangement  of  Mag- 
netic molecules  very  highly  mag-  netic  Molecules  in  highly  Magnetized  Iron. 

nified,  and  all  turned  in  the  same 

direction  so  that  the  two  ends  of  the  bar  are  magnet  poles.  The  dark 
ends  represent  the  south  poles  and  the  light  ends  the  north  poles  of  the 
molecules.  When  the  particles  are  arranged  with  their  like  poles  all 
pointing  in  the  same  direction  as  in  the  figure,  it  is  seen  that  the  poles 


74  ELECTRICITY  AND   MAGNETISM 

in  the  interior  of  the  material  are  facing  each  other  in  pairs,  north  to 
south,  and  must  therefore  neutralize  each  other's  effects,  but  unneutral- 
ized  poles  exist  at  the  ends  of  the  material. 

82.  Unit  Magnet  Pole.  —  In  order  to  know  the  strength  of  a  magnet, 
some  unit  of  measure  must  be  used  ;  and  so  we  say  theoretically  that  if  two 
small  similar  magnet  poles  placed  exactly  one  centimeter  (metric  meas- 
ure) apart  repel  or  push  each  other  with  a  force  equal  to  what  is  called 
a  Dyne,  they  are  magnet  poles  of  unit  strength  or  Unit  Poles.     In  speak- 
ing of  a  unit  pole  in  this  way,  it  is  supposed  that  the  pole  is  in  effect 
gathered  at  a  mathematical  point,  while  its  accompanying  pole  of  oppo- 
site sign  on  the  same  magnet  is  at  such  a  distance  as  to  be  unaffected  by 
the  attraction  and  repulsion.     The  condition  here  described  can  never 
be  physically  produced. 

83.  Force  exerted  between  Two  Magnet  Poles.  —  The  actual  force 
exerted  between  two  magnets  depends  upon  the  strength  of  their  poles 
and  their  distance  apart.     If  it  were  possible  to  have  two  separate  mag- 
net poles  of  small  size,  as  compared  with  their  distance  apart,  the  force 
exerted  between  them  would  be  equal  to  the  product  of  the  strengths  of 
the  poles  divided  by  the  square  of  their  distance  apart     This  is  similar 
to  the  law  of  the  force  exerted  between  two  small  isolated  bodies  holding 
electric  charges.1 

The  condition  required  for  the  law  of  force  to  be  fulfilled  can  only  be 
gained  by  using  poles  of  two  very  long,  thin  magnets.  The  force  be- 
tween two  actual  magnets  as  it  may  usually  be  measured  does  not  follow 
this  law  directly,  because  the  poles  are  of  considerable  size  as  compared 
with  their  distance  apart.  Every  small  portion  of  the  pole  of  one  mag- 
net exerts  a  force  on  every  small  portion  of  the  pole  of  the  other  magnet, 
in  accordance  with  the  law ;  and  when  all  these  small  forces  are  added 
together  the  law  is  apparently  changed,  though  it  is  based  on  the  funda- 
mental one. 

There  are  certain  similarities  which  may  be  perceived  between  the 
actions  of  magnets  and  of  charged  bodies,  but  there  are  also  marked 
differences,  so  a  close  relationship  is  not  evident.  There  is,  however, 
a  remarkably  close  relationship  between  magnetism  and  current  elec- 
tricity, which  will  be  described  in  a  later  chapter. 

1  Article  II. 


THE  NATURE   AND   PROPERTIES   OF   MAGNETISM 


75 


84.  Magnetic  Fields.  —  Any  open  space  in  which  there  is  magnetism 
and  consequently  magnetic  force,  is  called  a  Magnetic  Field  or  a  Mag- 
netic Field  of  Force.  The  magnitude  or  intensity  of  the  magnetic  force 
at  any  point  is  called  the  Strength  of  the  Field  at  that  point. 

If  an  independent  north  pole  could  be  placed  in  front  of  the  north 
pole  of  a  magnet,  it  would  be  repelled  by  the  latter  pole  and  be  attracted 
by  the  south  pole  of  the  magnet.  This  would  cause  the  independent 
pole  to  move  away  from  the  magnet's  north  pole  and  towards  its  south 
pole,  but  as  it  moved  it  would  continually  change  its  relative  distance 
from  the  two  poles,  and  the  relative  magnitude  of  the  forces  exerted  upon 
it  by  the  two  poles  would  vary.  The  direction  of  the  motion  of  the  inde- 
pendent pole  would  depend  upon  the  relative  direction  and  magnitude 
of  the  forces  which  the  two  poles  of  the  magnet  exerted  on  it  at  every 
point.  The  actual  path  would  be  a 
curved  line  very  much  like  the  line 
AB  in  Figure  42.  An  independent 
south  pole  would  move  in  an  opposite 
direction,  of  course,  but  over  a  similar 
path. 

As  already  explained,  it  is  impossible 
to  have  an  independent  magnet  pole, 

but  for  this  experiment  the  companion  pole  may  be  sufficiently  far 
removed  to  satisfactorily  show  the  action. 

A  shallow  glass  dish  containing  a  little  water  may  be  placed  over  a 
magnet  (Fig.  43).  By  properly  sticking  a  magnetized  sewing-needle 

in  a  cork,  it  may  be  floated 

Sr^^sT^ •  ,>     upon  the  water  in  a  vertical 

position  with  one  of  its  poles 
close    to   the    bottom   of   the 


FIG.  42.  —  A  Line  of  Force  set  up  by 
the  Magnet  NS. 


LS n  dish.      Then  the   upper  pole 

FIG.  43. -Experiment  to  illustrate  the  Movement     w\\\   be   SO   much  farther  away 
of  a  Free  Magnet  Pole  when  in  the  Presence      .. 

of  a  Bar  Magnet.  fro™    the    magnet    than    the 

lower  one  that  the  latter  will 

be  affected  by  the  force  due  to  the  magnet  almost  as  would  an  inde- 
pendent pole.  If  the  lower  pole  of  the  needle  is  a  north  pole,  it  will 
tend  to  move  through  the  water,  when  placed  in  front  of  the  north  pole 


ELECTRICITY   AND   MAGNETISM 


of  the  magnet,  in  a  curved  line  away  from  the  north  pole  and  toward 
the  south  pole.  If  the  lower  pole  of  the  needle  is  a  south  pole,  it 
will  tend  to  move  from  the  south  pole  toward  the  north  pole.  This 
is  exactly  as  already  explained  for  an  independent  magnet  pole.  The 
experiment  here  outlined,  and  which  may  be  so  readily  tried,  is  more 
striking  when  the  magnet  is  a  strong  electromagnet  such  as  will  be 
explained  later,  because  the  force  (acting  on  the  floating  needle  to  move 
it)  is  then  greater. 

The  direction  of  the  force  at  different  points  of  the  magnetic  field 
which  is  around  a  magnet  may  be  shown  by  another  simple  experiment. 
A  sheet  of  paper  may  be  laid  over  the  magnet  and  iron  filings  sifted 
over  it.  Now  if  the  paper  is  lightly  tapped  the  filings  will  arrange 
themselves  in  curved  lines  like  those  shown  in  Figure  39,  all  of  which 
converge  toward  the  two  poles.  If  the  figure  were  sufficiently  large  it 
would  be  approximately  shown  that  every  line  which  starts  out  from  one 
pole  finds  its  way  round  to  the  other  pole.  The  lines  of  iron  filings  may 
be  easily  fixed  in  position  if  the  paper  is  paraffined  before  using  it,  by 
simply  passing  the  flame  of  a  Bunsen  gas  burner  over  it.  This  softens 
the  paraffine  and  the  bits  of  iron  stick  fast. 

A  magnetic  field  exists  all  around  a  magnet  exactly  like  that  which  is 
shown  by  these  experiments  in  one  plane.  This  may  be  proved  by 
hanging  a  short  magnetized  sewing-needle  on  a  light  thread  and  bring- 
ing it  near  a  magnet  (Fig.  44). 
The  needle  will  take  a  position 
at  every  point  so  that  its  direc- 
tion is  tangent  to  the  direction 
which  a  line  of  iron  filings  would 
take  at  the  same  point.  The 
reason  for  the  needle  taking  this 
position  is  because  its  north  pole 
tends  to  go  one  way  and  its  south 
pole  the  other,  so  that  the  needle  turns  around  until  the  pull  on  the 
two  poles  is  in  a  direct  line  through  the  length  of  the  needle.  The 
iron  filings  used  in  the  experiment  described  above,  are  nothing  more 
than  little  magnets  created  by  induction,  and  they  take  up  their  position 
for  the  same  reason  that  the  needle  does. 


b  a 


FlG.  44.  —  Magnetic  Needle  used  to  explore 
Magnetic  Field  around  a  Magnet. 


THE  NATURE  AND   PROPERTIES   OF   MAGNETISM  77 

It  must  be  remembered  that  in  all  cases  of  attraction  or  repulsion 
between  two  bodies  the  force  exerted  is  mutual,  and  either  body  will  be 
moved  if  not  too  firmly  fixed.  This  is  true  whatever  be  the  cause  of  the 
force,  as  for  instance,  electrification,  magnetism,  gravity,  muscular  force, 
or  any  other  cause.  The  fact  that  the  action  is  mutual  may  be  proved 
by  placing  a  bit  of  iron  on  a  cork  floating  in  water  and  presenting  a 
small  magnet  to  it.  The  iron  will  be  attracted  by  the  magnet  and  the 
cork  will  be  moved  through  the  water  by  the  force  of  the  attraction. 
Now  if  the  magnet  is  placed  upon  the  cork  and  the  iron  is  brought  near 
it,  the  attraction  between  the  magnet  and  the  iron  will  again  move  the 
cork.  Finally,  if  the  iron  and  the  magnet  are  placed  on  separate  corks, 
the  corks  will  move  toward  each  other.  This  shows  that  the  force  is 
mutual,  and  it  is  also  possible  to  show  that  the  pull  on  the  iron  is  always 
equal  to  the  pull  on  the  magnet. 

85.  Lines  of  Magnetic  Force.  —  A  convenient  way  of  looking  upon  a 
magnetic  field  is  to  consider  it  a  space  which  is  more  or  less  filled  with 
lines  of  magnetic  force.     The  strength  of  field  may  be  represented  by 
the   number   of    Lines   of    Force    to    the    square   centimeter    (metric 
measure).     Then,  if  the  strength  of  the  field  be  such,  for  instance,  that 
a  unit  pole  when  placed  in  it  experiences  a  force  of  ten  dynes,  or  units 
of  force,  we  may  consider  the  field  as  having  ten  lines  of  force  per 
square  centimeter.     These  lines  of  force  no  more  actually  exist  than  do 
definite  stream  lines,  or  lines  of  flow,  exist  in  water  which  is  flowing 
around  in  a  tub,  but  the  idea  based  on  this  assumed  existence  is  a  very 
useful  and  practical  one.    The  directions  of  the  lines  of  force  are  traced 
out  by  the  iron  filings  as  shown  in  Figure  39,  or  the  path  of  travel  of  the 
supposed  free  magnet  pole  shown  in  Figure  42. 

86.  Position  which  a  Magnet  tends  to  take  in  a  Magnetic  Field .  — 
From  what  we  have  learned  from  the  mutual  action  of  magnets,  we  can 
now  see  that  when  a  magnet  is  placed  in  a  magnetic  field  it  apparently 
tends  to  set  itself  in  such  a  direction  that  its  own  lines  of  force,  where 
they  are  within  its  body,  are  parallel  with  the  lines  of  force  of  the  ex- 
ternal field.     The  effect  is  exactly  as  though  lines  of  force  tend  to  turn 
themselves  so  as  to  be  parallel  with  each  other  and  in  the  same  direction. 

87.  Unit   Magnetic    Field. — The  theoretical  method    of  measuring 
the  strength  of  a  magnetic  field  is  by  determining  the  force  which  it 


78  ELECTRICITY   AND   MAGNETISM 

exerts  upon  a  unit  magnet  pole.  A  unit  field  is  one  which  pushes  upon 
a  unit  pole  with  a  force  of  one  unit  called  a  dyne.  The  force  exerted  on 
a  magnet  pole  in  a  magnetic  field  depends  upon  the  strength  of  the  pole 
and  the  number  of  lines  of  force  for  every  square  centimeter  of  the 
field. 

A  uniform  field  is  one  which  pushes  upon  a  pole  equally  at  all  points. 
A  unit  field  is  conceived  to  have  one  line  of  force  for  each  square  centi- 
meter, but  it  must  be  remembered  that  this  arrangement  is  a  purely 
hypothetical  and  theoretical  conception. 

88.  Magnetic   Density   and   Flux. — The   magnetic  density  in    any 
magnet  or  magnetic  field  is  the  number  of  lines  of  force  passing  through 
each  square  centimeter  or  square  inch  of  cross  section.     When  a  mag- 
netic field  is  not  uniform,  the  density  varies  at  different  points,  but  in  a 
uniform  field  the  density  is  the  same  everywhere.     This  use  of  the  term 
"density"  is  not  quite  like  that  to  which  we  are  ordinarily  accustomed, 
but  the  meaning  need  not  be  misunderstood  on  that  account,  and  the 
phrase  Magnetic  Density  is  a  very  satisfactory  one. 

The  Magnetism  or  Magnetic  Flux  is  the  total  number  of  lines  of  force 
passing  through  the  magnet  or  the  part  of  a  field  considered.  For  in- 
stance, in  any  particular  magnet,  the  magnetic  density  in  the  magnet  is 
the  number  of  lines  of  force  passing  through  each  square  centimeter  of 
its  cross  section,  while  the  flux  in  the  magnet  is  the  total  number  of 
lines  passing  through  its  entire  cross  section. 

89.  Magnetomotive    Force.  —  There     is    evidently    some    Magnetic 
Pressure  or  difference  of  magnetic  level  between  the  two  poles  of  a  mag- 
net, which  tends  to  set  up  the  lines  of  force  between  the  two  points. 
This  is  called  Difference  of  Magnetic  Potential  or  Magnetomotive  Force, 
or  better,  Magnetic  Pressure.     An  analogy  is  seen  in  the  electrical  po- 
tential which  is  explained  in  Chapter  III.    The  stronger  a  magnet  is,  the 
stronger  is  the  magnetic  pressure  between  its  poles,  and  the  greater  is 
the  work  required  to  push  an  independent  north  pole  of  given  strength 
from  the  south  to  the  north  pole  of  the  magnet.      The  difference  of 
magnetic  potential  or  the  magnetomotive  force  between  the  poles  of  the 
magnet,  is  measured  by  the  work  required  to  push  a  //////  north  pole  from 
the  south  to  the  north  end  of  the  magnet,  —  just  as  a  mechanical  poten- 
tial or  "  head  "  may  be  measured  by  the  work  required  in  pushing  against  it. 


THE  NATURE  AND   PROPERTIES  OF  MAGNETISM  79 

90.  Terrestrial  Magnetism.1  —  In  Article  70  it  is  stated  that  the 
north  pole  of  a  magnetic  needle  tends  to  point  toward  the  north  and 
also  dips  down  somewhat  towards  the  earth  if  permitted  to  do  so.  This 
indicates  that  the  earth  itself  acts  like  a  great  magnet  having  its  negative 
magnetic  pole  somewhere  near  the  region  of  the  geographical  north  pole. 
The  earth's  magnetic  poles  are  probably  of  large  surface  and  irregular, 
just  as  is  often  relatively  the  case  in  an  iron  magnet,  and  these  are  not 
exactly  at  the  geographical  poles.  And  so  a  magnetic  needle  will  not 
point  exactly  north  and  south.  This  difference  from  a  true  north  and 
south  position  is  called  the  Declination  of  the  magnetic  needle,  and  it 
varies  from  place  to  place.  The  amount  the  needle  dips  from  the  hori- 
zontal is  called  the  Inclination,  and  it  is  zero  near  the  magnetic  equator 
and  ninety  degrees  directly  over  the  earth's  magnetic  pole.  The  strength 
of  the  earth's  magnetic  field  at  any  place  is  called  its  Intensity. 

Each  of  these  "  elements  "  of  the  earth's  magnetism  varies  from  place 
to  place  on  the  earth's  surface,  and  in  addition  to  that  varies  more  or 
less  regularly  from  year  to  year,  as  though  the  earth's  magnetic  strength 
varied  and  the  positions  of  its  poles  moved.  The  gradual  change  of 
the  magnetic  declination  as  time  goes  on  changes  the  compass  "  bear- 
ings "  which  determine  property  lines  in  land  surveying,  and  surveyors 
must  carefully  keep  this  in  mind  when  they  undertake  to  seek  out 
division  lines  which  are  described  in  old  deeds  by  their  "  bearings." 

Various  local  disturbances  may  cause  local  variations  of  the  compass, 
so  that  it  does  not  point  in  the  proper  direction.  Wherever  iron-bear- 
ing rocks  and  sands  are  plentiful,  these  local  variations  are  likely  to 
occur;  and  what  are  called  magnetic  storms  also  cause  temporary 
disturbances  of  compasses,  but  the  effects  of  these  usually  pass  away 
after  a  few  days.  The  cause  of  magnetic  storms  is  unknown,  though  it 
seems  to  be  connected  in  some  way  with  great  disturbances  in  the  solar 
system. 

It  is  interesting  to  know  that  Columbus  made  the  discovery  that  the 
magnetic  declination  differed  at  different  places,  when  on  his  first 
voyage  to  America.2  Before  that  time  it  had  been  supposed  that  the 
compass  needle  everywhere  pointed  to  the  same  spot,  though  it  was 
known  to  deviate  from  the  true  north.  The  discovery  that  the  declina- 

1Also  see  Article  71.  2  Article  71. 


80  ELECTRICITY  AND   MAGNETISM 

tion   of  the   needle   was   variable   caused   great   excitement  and  fear 
amongst  Columbus's  crew. 

The  reason  that  the  earth  is  a  great  magnet  we  do  not  know,  but  its 
magnetic  effects  have  been  studied  and  its  magnetic  strength  and  the 
locations  of  its  poles  have  been  determined.  Magnetic  maps  have  also 
been  made  which  show  the  usual  declination  and  inclination  of  the 
magnetic  needle  at  the  different  points  on  the  earth's  surface.  Such 
maps  may  be  used  for  correcting  the  declinations  of  a  compass  where 
local  variations  do  not  occur. 

QUESTIONS 

1.  Can  you  tell  what  magnetism  is? 

2.  Why  is  magnetism  supposed  to  be  molecular  in  nature? 

3.  What  is  Coulomb's  theory  of  magnetism? 

4.  What  is  Ampere's  theory  of  magnetism  ? 

5.  When  did  Weber  advance  his  theory  of  magnetism? 

6.  What  is  Weber's  theory  of  magnetism? 

7.  What  is  a  unit  magnet  pole? 

8.  What  is  the  law  of  attraction  between  two  magnet  poles? 

9.  Suppose  two  long  magnets  with  their  poles  concentrated  into  points,  of  20 
units  strength  each,  which  have  their  unlike  poles  2  centimeters  apart,  what  force  is 
exerted  between  their  two  poles?  Ans.  100  dynes. 

10.  Suppose    two    poles   as    in   Question  9,  but  separated   I  centimetre,  one  of 
them  being  of  one  unit  strength.     If  the  force  between  them  is  10  dynes,  what  is  the 
strength  of  the  second  pole?  Ans.  10  units. 

1 1.  Suppose  two  poles  of  the  same  strength  as  in  Question  9.    If  the  force  exerted 
between  the  poles  is  25  dynes,  how  far  are  they  apart?  Ans.  4  cm. 

12.  Can  a  magnet  pole  be  confined  to  a  point? 

13.  What  is  a  magnetic  field? 

14.  What  is  strength  of  field? 

15.  What  would  be  the  general  form  of  the  path  of  a  free  north  pole  if,  after 
being  placed  near  the  north  pole  of  a  magnet,  it  were  allowed  to  move? 

1 6.  Describe  a  number  of  ways  by  which  the  extent  and  direction  of  a  field  of 
force  may  be  determined. 

17.  If  a  magnet  pole  is  brought  near  a  piece  of  iron,  will  the  pole  exert  a  pull  on 
the  iron?     Will  the  iron  exert  a  pull  on  the  pole?     Will  the  two  pulls  be  equal? 

1 8.  Describe  an  experiment  that  will  show  that  the  force  of  attraction  or  repul- 
sion between  two  bodies  is  mutual. 

19.  What  are  lines  of  force? 


THE  NATURE   AND    PROPERTIES  OF   MAGNETISM  8 1 

20.  If  a  portion  of  a  field  is  of  10  units  strength,  how  can  you  represent  it? 

21.  How  many  lines  of  force  per  square  centimeter  cross  section  are  there  con- 
ceived to  exist  in  a  field  of  25  units  strength? 

22.  What  is  the  general  form  of  the  lines  of  force  about  a  bar  magnet? 

23.  If  a  magnet  is  suspended  in  a  magnetic  field,  how  will  it  tend  to  place  itself  ? 

24.  What  is  a  unit  field? 

25.  How  many  lines  of  force  per  square  centimeter  cross  section  are  there  in  a 
field  of  unit  strength? 

26.  What  is  a  uniform  field? 

27.  If  a  pole  of  10  units  strength  is  placed  in  a  field  of  5  units  strength,  with 
what  force  will  it  be  acted  upon?  Ans.  50  dynes. 

28.  If  a  pole  of  5  units  strength  be  acted  upon  by  a  force  of  20  dynes,  how  strong 
is  the  field?  Ans.  4  units. 

29.  If  a  field  having  10  lines  of  force  per  square  centimeter  cross  section  acts 
upon  a  pole  with  a  force  of  2  dynes,  how  strong  is  the  pole?  Ans.  \  unit. 

30.  What  is  magnetic  density? 

31.  What  is  magnetic  flux? 

32.  Is  the  -magnetic  density  the  same  at  all  points  in  the  field  set  up  by  a  bar 
magnet? 

33.  What  is  magnetomotive  force,  or  magnetic  pressure? 

34.  Compare  magnetic  pressure  with  electric  pressure. 

35.  When  is  there  a  unit  magnetic  pressure  between  two  points? 

36.  Must  work  be  done  to  move  a  north  pole  from  the  south  to  the  north  pole  of 
a  magnet? 

37.  Will  work  be  done  by  a  south  pole  when  it  moves  from  the  south  pole  to  the 
north  pole  of  a  magnet? 

38.  If  a  north  pole  is  moved  from  between  two  points  in  a  field  against  the  lines 
of  force,  will  the  work  done  upon  it  be  equal  to  that  which  the  pole  will  do  when  it 
moves  with  the  lines  offeree  between  the  same  two  points? 

39.  How  much  more  work  will  be  required  to  move  a  north  pole  between  two 
points  if  the  difference  of  pressure  is  increased  to  double  its  original  value  ? 

40.  If  a  very  short  magnet  is  placed  in  a  magnetic  field,  will  it  tend  to  move 
bodily  along  the  lines  offeree?     Why  not? 

41.  To  what  may  the  magnetic  condition  of  the  earth  be  compared? 

42.  Which  is  the  positive  magnetic  pole  of  the  earth? 

43.  What  is  magnetic  declination? 

44.  What  is  magnetic  inclination? 

45.  What  is  the  strength  of  the  earth's  magnetic  field  at  any  point  called? 

46.  Are  declination,  inclination,  and  intensity  constant  at  all  points  of  the  earth's 
surface? 

47.  What  variation  of  the  earth's  magnetic  elements  affects  survey  bearings? 

48.  Tell  what  you  can  about  "  local  variations  "  and  *'  magnetic  storms." 

G 


CHAPTER   VII 

ELECTRIC   CIRCUITS   AND  THE   FLOW   OF   ELECTRICITY; 
OHM'S   LAW 

91.  Conductivity.  —  We  know  by  experience  that  the  amount  of 
energy  required  to  propel  water  through  a  pipe  depends  upon  the  size 
of  the  pipe  and  its  construction.  Also,  in  two  pipes  of  the  same  size,  if 
one  has  a  rough  inner  surface  and  the  other  a  smooth  one,  we  know  that 
the  former  is  the  poorer  conductor  of  the  water.  Although  electricity 
is  not  a  fluid,  the  analogy  between  its  flow  and  that  of  water  is  in  many 
ways  close.  The  flow  of  electricity  is  also  known  by  experience  to  be 
dependent  upon  the  dimensions  of  the  conductor  and  the  material  from 
which  it  is  made.  The  electricity  may  be  considered  to  flow  through 
the  entire  cross  section  of  the  conductor,  so  that  any  resisting  action 
is  uniform  throughout  the  material  instead  of  being  a  "  skin  "  or  friction 
effect,  as  in  the  case  of  water  flowing  in  a  pipe.  The  relative  powers 
of  different  materials  for  conducting  electricity  are  called  their  Con- 
ductivities. 

A  table  is  given  in  Article  7  which  shows  the  comparative  order  of  the 
conducting  powers  of  various  materials.  It  is  seen  that  the  metals  stand 
at  the  head  of  the  list,  and  their  conducting  power  is  so  much  better 
than  that  of  other  materials  that  we  ordinarily  speak  of  them  alone  as 
the  conductors  of  electricity  or  Electrical  Conductors.  Amongst  the 
pure  metals  themselves  there  is  considerable  difference  in  conducting 
power,  while  mixing  impurities  in  metals  or  mixing  them  together  gen- 
erally decreases  their  conducting  power.  The  following  table  gives 
a  number  of  the  better  known  metals  and  common  alloys  in  the  ap- 
proximate order  of  their  conducting  powers.  The  figures  at  the  right 
hand  of  the  names  of  the  metals  show  the  average  relative  conducting 
powers  of  pure  metals  and  of  alloys  of  fixed  composition,  in  percentages 
of  the  conducting  power  of  pure  silver.  Pure  silver  and  pure  copper 

82 


ELECTRIC  CIRCUITS  AND  THE   FLOW  OF  ELECTRICITY        83 

vie  with  each  other  for  place  as  the  best  conductor  known,  and  no  other 
metals  approach  them  very  closely.  Aluminum  is  so  very  light  in  weight 
that  pure  aluminum  conductors  have  even  less  resistance  than  copper 
conductors  of  equal  length  and  weight ;  but  the  relative  conducting 
powers  or  conductivities  which  are  presented  in  the  table  refer  to  con- 
ductors of  equal  lengths  and  cross  sections. 

Silver  .  .   100     Aluminum  .  .  55     Wrought  Iron  .  .   16   Lead    ...  8 

Copper  .  100     Zinc 28     Nickel 12   Cast  Iron  .  3 

Gold    .  .     75     Platinum    .  .  17     Tin 12   Mercury.  1.6 

Platinum  Silver  made  of  2  parts  Platinum  and  i  part  Silver  .  .  .  .  .  6.4 
German  Silver  made  of  5  J-  parts-Copper,  2  parts  Zinc,  2\  parts  Nickel  3.5 
German  Silver  made  of  6  parts  Copper,  2^  parts  Zinc,  i  \  parts  Nickel  5. 
German  Silver  made  of  5  parts  Copper,  3!  palts  Zinc,  ii  parts  Nickel  7.5 

The  quality  of  a  metal  and  the  way  in  which  it  has  been  handled  in 
the  course  of  manufacture  affect  the  conducting  power  to  a  consid- 
erable degree.  Pure  copper  that  comes  from  the  ore  of  the  Lake  Supe- 
rior copper  mines  or  the  Montana  mines  appears,  as  a  rule,  to  have 
a  little  higher  conductivity  than  that  coming  from  the  Arizona  mines. 
Annealed  metals  (that  is,  metals  which  have  been  softened  and  tough- 
ened by  properly  cooling  from  a  high  temperature)  generally  have  a 
slightly  greater  conductivity  than  hardened  metals,  and  wrought  metals 
than  cast  metals. 

92.  Ohm's  Law.  —  When  water  is  forced  through  a  pipe  under  press- 
ure from  a  pump  or  other  source  of  pressure,  the  stream  of  water  which 
flows  is  proportional  to  the  pressure  divided  by  the  frictional  resistance 
which  the  pipe  presents  to  the  flow  of  the  water.  In  the  same  way, 
when  a  current  of  electricity  flows  through  a  wire  under  the  pressure 
from  a  battery  or  other  source  of  electricity,  the  current  which  flows  in 
the  circuit  is  equal  to  the  pressure  divided  by  the  resistance  of  the  circuit. 
This  relation  between  electric  current,  pressure,  and  resistance  is  called 
Ohm's  Law,  after  the  name  of  the  German  scientist  who  first  (in  1827) 
formally  announced  it.  The  relation  representing  Ohm's  Law  is  often 
written 


84  ELECTRICITY   AND   MAGNETISM 

where  C,  E,  and  R  stand  for  current,  pressure,  and  resistance.  This  is 
a  very  good  form  in  which  to  commit  the  relation  to  memory.  The 
expression  as  written  may  be  read  C  equals  E  divided  by  R. 

From  the  relation  as  written  above  it  is  evident,  also,  that  E  equals  C 
times  R,  and  R  equals  E  divided  by  C.  Consequently  if  any  two  out 
of  the  three  fundamental  electrical  quantities  whi'ch  exist  in  a  circuit  are 
given,  the  third  can  at  once  be  calculated.  Thus,  if  a  16  candle  power 
incandescent  lamp  is  known  to  take  -|-  an  ampere  when  connected  to 
a  circuit  which  furnishes  current  at  a  pressure  of  no  volts,  the  resistance 
of  the  lamp  when  in  operation  may  be  calculated  at  once  to  be  no 
divided  by  ^,  which  gives  the  resistance  as  220  ohms. 

93.  The  Ohm.  —  Ohm  is  the  name  of  the  unit  in  which  electrical  re- 
sistance is  measured,  as  pound  is  the  name  of  the  unit  in  which  weight 
is  measured.     The  word  "  ohm  "  is  taken  from  the  name  of  the  German 
scientist,  Dr.  Ohm,  who  first  set  forth  the  law  of  electric  flow  as  told 
above.     Ampere   (from   the  name  of  a  great  French  scientist)   is  the 
name  of  the  unit  in  which  electric  current  is  measured,  as  has  already 
been  explained  in  Article  22  ;  and  volt  (from  the  name  of  a  great  Italian 
scientist)  is  the  name  of  the  unit  in  which  electric  pressure  is  measured, 
as  has  been  explained  in  Article  23,, 

94.  Effect  of  Internal  Resistance.  —  In  the  example  given  above,  we 
have  assumed  that  the  source  of  electricity  has  sufficient  capacity  to 
keep  up  the  full  pressure  at  the  lamp  terminals  when  current  is  flowing 
through  the  lamp.     Sometimes  this  is  not  the  case  on  account  of  the 
resistance  to  be  found  in  the  source  itself,  or  the  Internal  Resistance 
of  the  source.     A  similar  condition  is  frequently  met  when  a  pump  is 
attached  to  a  large  hose.     When  the  hose  nozzle  is  partly  closed,  the 
pump  will  give  a  large  pressure ;  but  when  the  nozzle  is  opened,  the 
pressure  falls  because  the  pump  does  not   have  sufficient  capacity  to 
keep  up  the  supply. 

When  it  is  desired  to  determine  the  current  that  will  flow  through  a 
circuit  due  to  a  pressure  from  a  source  of  current  that  has  an  appreciable 
internal  resistance,  it  is  necessary  to  add  up  the  resistances  of  all  parts 
of  the  circuit  before  making  the  calculation.  For  instance,  if  two  cells 
of  battery,  each  giving  a  pressure  of  i.i  volts,  and  each  having  an 
internal  resistance  of  3  ohms,  are  connected  in  series  with  an  external 


ELECTRIC  CIRCUITS   AND   THE   FLOW   OF   ELECTRICITY        85 

circuit  of  2.8  ohms  resistance,  then  the  total  resistance  in  the  circuit  is 
3  plus  3  plus  2.8,  or  8.8,  and  the  pressure  which  acts  to  cause  current 
to  flow  through  the  circuit  is  2.2  volts.  The  current  flowing  under  these 

/  £  I         2  2\ 

circumstances  is  \  ampere  (  C=— ,  or  -  =  —-]. 

\^          /?          4       8.8y 


PROBLEMS 

A.  If  a  wire  of  10  ohms  resistance  has  a  pressure  of  20  volts  impressed  upon  its 
terminals,  what  current  will  flow?     Ans.    2  amperes. 

B.  What  is  the  hot  resistance  of  a  lamp  filament  which  uses  .5  of  an  ampere  at 
100  volts?     Ans.    200  ohms. 

C.  If  a  battery  cell  sets  up  at  its  terminals  a  pressure  of  2  volts  when  on  open  cir- 
cuit, what  is  its  internal  resistance,  if  the  pressure,  measured  between  the  terminals, 
falls  to  1 1  volts  when  2  amperes  are  flowing?     (Aid:  ^  volt  is  used  in  forcing  the 
current  through  the  cell.)     Ans.    J  ohms. 

D.  A  lamp  filament  has  a  hot  resistance  of  6  ohms,  and  requires  I  ampere  to 
bring  it  to  proper  incandescence.     How  many  battery  cells  in  series,  having  an  open 
circuit  pressure  of  2  volts  and  an  internal  resistance  of  \  ohm  each,  will  be  required 
for  operating  the  lamp  ?     Ans.  4  cells. 

E.  How  much  pressure  is  generated  by  a  battery  which  has  an  internal  resistance 
of  8  ohms,  if  when  it  is  short-circuited  by  a  wire  of  negligible  resistance  a  current  of 
2  amperes  flows?     Ans.    16  volts. 

F.  If  the  cells  of  Example  E  are  in  series  and  generate  2  volts  each,  what  resist- 
ance has  each  cell  ?     Ans.    I  ohm. 

G.  A  certain  piece  of  wire  has  an  "  insulation  resistance  "  measured  through  the 
insulating  covering  between  the  conductor  and  the  ground  of   500,000  ohms;   how 
much  pressure  would  cause  a  current  of  one  thousandth  of  an  ampere  to  leak  from 
it  ?     Ans.    500  volts. 

H.  If  a  battery  of  five  gravity  cells,  each  of  which  gives  a  pressure  of  1.08  volts, 
and  has  an  internal  resistance  of  4  ohms,  is  connected  in  series  with  an  external  resist- 
ance of  7  ohms,  what  current  flows  through  the  circuit  ?  Ans.  -^  amperes. 

/.  If  two  cells  which  respectively  give  pressures  of  1.8  volts  and  1.08  volts  are 
connected  to  a  circuit  in  opposition  (that  is,  with  their  poles  connected  so  that  they 
tend  to  send  currents  in  opposite  directions),  and  a  current  of  .4  amperes  flows,  how 
much  current  will  flow  if  the  cells  are  connected  to  the  same  circuit  properly  in 
series?  Ans.  1.6  amperes. 

95.  The  Standard  of  Resistance.  —  The  resistance  to  the  flow  of  water 
through  a  pipe,  as  said  before,  is  a  surface  or  "  skin  "  friction  effect,  and 
depends  upon  the  velocity  with  which  the  water  flows,  the  number  and 


86  ELECTRICITY  AND   MAGNETISM 

form  of  bends  in  the  pipe,  the  form  of  its  cross  section,  and  its  length. 
The  true  electrical  resistance  of  a  conductor  is  quite  different  from  this, 
since  it  simply  depends  upon  the  nature  of  the  metal  from  which  the 
conductor  is  made,  the  area  of  its  cross  section,  its  length,  and  its 
temperature. 

The  greater  the  cross  section  of  a  conductor  the  greater  is  its  electrical 
conducting  power,  and  therefore  the  less  is  its  resistance;  and  the  longer 
the  wire  the  less  is  its  conducting  power,  and  therefore  the  greater  is  its 
resistance.  The  cross  sections  of  ordinary  cylindrical  wires  are  pro- 
portional to  the  squares  of  their  diameters,  and  consequently  the  con- 
ducting powers  of  such  wires  of  equal  length  are  directly  proportional 
to  the  squares  of  their  diameters.  This  makes  the  resistance  of  similar 
wires  vary  inversely  as  the  squares  of  their  diameters.  For  instance,  if 
a  certain  copper  wire  has  a  resistance  of  one  ohm,  the  resistance  of  a 
copper  wire  of  the  same  length  but  of  twice  the  diameter  is  only  one- 
fourth  of  an  ohm,  since  the  square  of  two  is  four. 

The  adopted  definition  of  the  value  of  the  ohm  is  based  upon  this 
property  of  electrical  resistance,  which  depends  simply  upon  the  nature  of 
the  metal  conductor,  its  temperature,  its  length,  and  the  inverse  of  its 
cross  section.  The  approved  definitions  of  all  the  electrical  units  were 
adopted  at  the  Electrical  Congress  held  in  Chicago  in  August,  1893. 
The  definition  of  the  unit  of  resistance  makes  one  ohm  equal  to  the 
resistance  of  a  column  of  pure  mercury  which  is  io6.j  centimeters  long, 
which  has  a  uniform  cross  section,  and  which  contains  14.4521  grammes 
of  mercury ;  the  temperature  being  that  of  melting  ice.  This  gives  to  the 
column  a  uniform  cross  section  of  one  square  millimeter  (metric  sys- 
tem). The  ohm  as  thus  defined  is  called  the  International  Ohm,  to 
distinguish  it  from  units  based  on  definitions  adopted  at  previous  elec- 
trical congresses,  and  which  differ  slightly  from  the  international  ohm 
and  from  one  another,  exactly  as  different  kinds  of  quart  measures  differ 
from  one  another,  as  is  told  in  books  on  arithmetic.  It  is  generally 
believed  that  the  definitions  given  by  the  Chicago  Electrical  Congress 
will  be  universally  accepted  and  will  never  be  changed.  The  units  by 
which  electricity  is  measured  will  then  be  the  same  in  all  countries. 
This  is  true  of  no  other  units  which  are  used  in  common  measurements. 

• 

Since  a  column  of  mercury  is  an  inconvenient  device  to  handle, 


ELECTRIC  CIRCUITS  AND  THE   FLOW  OF  ELECTRICITY        S/ 

standard  resistances  made  of  mercury  are  not  used  in  ordinary  measure- 
ments of  electrical  resistance,  but  coils  of  German  silver  wire,  or  other 
wires  of  high  resistance,  are  used.  These  coils  are  carefully  adjusted  in 
resistance  to  a  desirable  number  of  ohms,  and  they  can  then  be  used 
in  the  measurement  of  the  resistance  of  any  conductor  by  methods 
which  will  be  explained  on  later  pages.  Mercury  resistances  are  used 
only  in  well-equipped  scientific  laboratories  to  determine  the  real  resist- 
ances of  the  common  wire  Resistance  Coils. 

96.  The  Standard  of  Current.  —  Before  the  Chicago  Electrical  Con- 
gress was  held,  the  fundamental  definition  of  the  ampere  had  usually 
been  based  upon  the  electromagnetic  effects  of  currents;  but  at  that 
Congress   a   definition    was    adopted   which    is    based  on  the   electro- 
chemical effects  of  currents,  which  are  explained  in  Chapter  V.     The 
International   Ampere   as    thus    defined    is   the   steady   current   which 
deposits  silver  at  the  rate  of  .001118  grammes  per  second  from  a  solu- 
tion of  silver  nitrate  in  water,  the  solution  being  of  a  given  fixed  strength 
to  ensure  regular  action. 

Measurements  of  electrical  currents  in  practical  tests  are  more  fre- 
quently made  by  means  of  instruments  depending  upon  the  magnetic 
effects  of  the  currents  than  according  to  the  means  indicated  in  the 
definition  of  the  ampere.  Methods  of  measurement  based  on  the 
electrochemical  effects  of  currents  are  very  valuable  for  determining 
whether  the  indications  of  electromagnetic  instruments  are  correct. 

97.  The  Standard  of   Pressure.  —  In   order  that  the  fixed  relation 

represented  by  Ohm's  Law  (  Current  =  =-  -  )  shall  hold  with  these 

\  Resistancey 

definitions,  the  International  Volt  as  defined  by  the  Chicago  Con- 
gress is  the  pressure  which  causes  a  current  of  one  ampere  to  flow 
through  a  resistance  of  one  ohm. 

98.  Effect  of  Temperature  on  the  Resistance  of  Materials.  — Reference 
has  already  been  made  to  the  effect  of  temperature  on  the  resistance  of 
metals.     The  resistance  of  most   metals  increases  as  the  temperature 
rises,  but  in  the  case  of  a  few  alloys  the  resistance  falls  very  slightly  as 
the  temperature  increases.     The  resistance  of  carbon  falls  quite  rapidly 
as  the  temperature  rises,  and  this  fall  is  sufficiently  great  to  reduce  the 
working  resistance,  or  Hot  Resistance,  of  an  incandescent  lamp  filament 


88 


ELECTRICITY   AND    MAGNETISM 


to  only  about  one-half  the  resistance  which  it  has  when  at  the  usual 
atmospheric  temperature.  The  resistance  of  liquids  and  of  most  insu- 
lating materials,  as  far  as  they  are  measurable,  also  decreases  as  the 
temperature  rises.  This  decrease  is  so  marked  in  some  insulating 
materials  (such  as  glass,  for  instance)  that  they  actually  become  con- 
ductors when  they  are  heated  red-hot  or  when  they  are  melted.  The 
operation  of  the  new  "  Nernst  lamps  "  depends  upon  this  characteristic. 

The  resistance  of  most  pure  metals  seems  to  change  at  approximately 
the  same  rate  ;  namely,  about  .4  of  i  per  cent  per  degree  of  the  centi- 
grade thermometer  scale,  or  .22  of  i  per  cent  per  degree  of  the 
Fahrenheit  thermometer  scale.  (One  degree  of  the  centigrade  scale  is 
equal  to  f  of  a  degree  of  the  Fahrenheit  scale.)  This  is  a  fairly  accu- 
rate value  of  the  Temperature  Coefficient  of  ordinary  copper.  A  change 
of  .4  of  i  per  cent  per  centigrade  degree  means  a  change  of  i  per  cent 
in  resistance  up  or  down  for  every  z\  degrees  centigrade  when  the  tem- 
perature varies  up  or  down.  This  is  also  nearly  equivalent  to  i  per  cent 
for  every  4.5  degrees  of  the  Fahrenheit  or  common  thermometer  scale. 

The  temperature  coefficient  of  alloys  depends  very  much  upon  the 
composition  of  the  mixture.  In  general,  German  silver  may  be  taken 
to  have  a  temperature  coefficient  about  one-tenth  as  great  as  that  of 
copper.  The  temperature  coefficients  of  the  alloys,  whose  comparative 
conductivities  are  given  in  the  first  part  of  this  chapter,  are  compared 
below  with  that  of  copper  :  — 


Cent. 

Fahr. 

Copper           

.40 

.22 

Platinum  Silver      

.031 

.017 

German  Silver  *     ....... 

•°33 

.018 

German  Silver  *     

.036 

.O2O 

German  Silver  1     

.040 

.022 

The  two  columns  of  figures  in  this  table  show  the  approximate  tem- 
perature coefficients  of  the  metals  expressed  as  the  percentage  changes 
of  resistance  per  degree  centigrade  and  Fahrenheit. 

1  Article  91. 


ELECTRIC  CIRCUITS  AND  THE  FLOW  OF  ELECTRICITY        89 


QUESTIONS 

1.  Compare  the  conductivity  of  an  electrical  conductor  to  the  conductivity  of  a 
water  pipe. 

2.  What  effect  has  the  alloying  of  metals  upon  their  conductivity  ? 

3.  What  are  the  relative  conductivities  of  copper,  wrought  iron,  zinc,  mercury, 
and  German  silver  compared  with  silver? 

4.  How  does  annealing  affect  the  conductivity  of  metals? 

5.  What  is  Ohm's  Law? 

6.  When  was  Ohm's  Law  advanced? 

7.  Compare  the  relations  shown  in  Ohm's  Law  to  those  of  water  flowing  through 
pipes. 

8.  What  is  an  ohm? 

9.  After  whom  were  the  ohm,  ampere,  and  volt  named? 

10.  If  a  battery  is  connected  to  an  electric  lamp,  what  effect  upon  the  pressure  sup- 
plied to  the  lamp  have  the  connecting  wires  and  the  internal  resistance  of  the  battery 
itself  ? 

11.  WThat  elements  determine  the  resistance  of  a  wire? 

12.  What  effect  has  the  cross  section  upon  the  resistance  of  a  wire? 

13.  What  effect  has  length  upon  the  resistance  of  a  wire? 

14.  Why  are  the  resistances   of  cylindrical  wires  of  like  metals  inversely  pro- 
portional to  the  squares  of  their  diameters? 

15.  What  effect  has  temperature  upon  the  resistance  of  most  metals? 

1 6.  Define  the  international  ohm. 

17.  When  and  where  was  the  international  ohm  adopted? 

1 8.  How  are  standards  of  resistance  made? 

19.  Define  the  international  ampere. 

20.  On  what  principle  do  current  measuring  instruments  usually  depend  ? 

21.  Define  the  international  volt. 

22.  What  is  the  rough  relation  between  the  hot  and  cold  resistances  of  a  carbon 
lamp  filament  ? 

23.  What  effect,  ordinarily,  has  heat  upon  the  resistance  of  liquids  and  insulators? 

24.  About  how  much  does  the  resistance  of  copper  increase  for  each  degree  centi- 
grade rise  in  temperature? 

25.  About  how  many  degrees  Fahrenheit  change  in  temperature  will  change  the 
resistance  of  copper  by  one  per  cent? 

26.  What  is  a  temperature  coefficient  ? 

27.  Do  pure  metals  have  the  same  or  different  temperature  coefficients? 

28.  About  what  is  the  temperature  coefficient  of  German  silver  compared  with 
copper  ? 

99.    The  Circular  Mil.  —  In  the  practical  measurement  of  wires  it  is 
usual  to  use  feet  in  measuring  the  length  and  Circular  Mils  in  measur- 


90  ELECTRICITY  AND   MAGNETISM 

ing  the  cross  section.  The  length  of  one  thousandth  of  an  inch  is 
called  a  Mil,  and  a  round  wire  one  mil  in  diameter  is  said  to  have  a 
cross  section  of  one  circular  mil.  The  cross  sections,  or  areas,  of  wires 
are  measured  in  this  unit;  and  it  is  a  very  convenient  unit  for  this 
reason :  the  areas  of  circles  are  proportional  to  the  squares  of  their 
diameters  —  consequently,  if  the  area  of  a  wire  one  mil  in  diameter  is 
called  a  circular  mil,  all  other  round  wires  have  an  area  or  cross  section 
which,  in  circular  mils,  is  numerically  equal  to  the  squares  of  their 
diameters.  A  circle  one  inch  in  diameter  is  one  thousand  mils  in 
diameter,  and  there  are  therefore  one  million  circular  mils  in  its  area. 

As  a  square  inch  has  an  area  ^  times  the  area  of  such  a  circle,  there 

7T 

must  be  -  X   1,000,000   (1,273,000)    circular  mils  in  a  square  inch. 

7T 

The  symbol  TT  (Greek  letter  pi)  is  used  to  represent  a  constant  value  of 
3.1416,  which,  in  inches,  is  equal  to  the  length  of  the  circumference 
of  a  circle  which  is  one  inch  in  diameter. 

100.  Specific  Resistance.  —  In  order  to  find  the  resistance  of  a  wire 
by  calculation,  it  is  necessary  to  know  the  resistance  of  a  piece  having 
a  unit  length  and  cross  section,  that  is,  the  resistance  of  a  wire  one  foot 
long  which  has  a  cross  section  of  one  circular  mil.     This  may  be  called 
the  Specific  Resistance  of  the  material  or  the  resistance  of  a  Mil  Foot. 
The  resistance  of  a  mil  foot  of  good  commercial  copper  is  very  nearly 
10.5  ohms  at  a  temperature  of  75°  F.     In  scientific  writings,  specific 
resistance  is  usually  given  on  the  basis  of  one  centimeter,  as  the  unit  of 
length,  and  one  square  centimeter  as  the  unit  of  cross  section,  instead 
of  on  the  basis  of  the  mil  foot  which  is  commonly  used  in  practice. 

101.  Determination  of  the  Resistance  of  a  Wire.  —  As  has  just  been 
said,1  the  resistance  of  a  wire  or  other  piece  of  any  particular  metal 
depends  directly  upon  its  length  and  inversely  upon  its   cross  section. 
So,  if  the  resistance  of  a  mil  foot  of  the  wire  (given  in  ohms)  is  multi- 
plied by  the  total  length  in  feet  and  divided  by  its  cross  section  in  circular 
mils,  the  result  will  be  the  resistance  of  the  whole  wire  in  ohms. 

This  may  be  expressed,  for  convenience,  in  this  way, 

R=X— , 
c.m.' 

1  Article  95. 


ELECTRIC  CIRCUITS  AND   THE   FLOW   OF   ELECTRICITY        91 

where  R  is  the  resistance  in  ohms,  X  the  resistance  of  a  mil  foot  (which 
is  10.5  ohms  for  copper  at  a  temperature  of  75°  F.),  Z  the  length  in! 
feet,  and  c.m.  the  circular  mils  in  the  cross  section. 

Now,  suppose  we  wish  to  find  the  resistance  of  a  copper  wire  1000 
feet  long  and  50,000  c.m.  in  cross  section,  the  following  expression  will 
result  from  the  above  reasoning  :  — 

1000 
R  =  10.5 =  .21, 

•*  50000 

and  the  resistance  of  the  given  wire  is  .21  of  an  ohm. 

The  specific  resistance  of  conductors  varies  greatly,  as  will  be  seen 
by  referring  to  the  table  in  Article  91,  where  the  relative  conductivities 
of  various  metals  are  given.  The  specific  resistance  of  any  of  the 
materials  given  in  the  table  may  be  found  by  comparing  with  that  of 
copper.  For  instance,  if  it  is  desired  to  find  the  specific  resistance 
of  lead  in  mil  feet,  we  would  divide  100  by  8,  which  equals  12.5,  and 
multiply  this  by  10.5  (the  resistance  of  a  mil  foot  of  copper),  which 
shows  that  the  value  sought  is  131.25  ohms. 

PROBLEMS 

A.  What  is  the  resistance  at  ordinary  temperature  of  a  copper  wire  2500  ft.  long 
and  having  a  cross  section  of  10,500  circular  mils?     Ans.    2.5  ohms. 

B.  Suppose  it  is  desired  to  have  a  copper  wire  of  .5  ohms  resistance  and  2000 
ft.  long;   what  must  be  its  cross  section?     Ans.    42,000  circular  mils. 

C.  If  it  is  required  to  transmit  10  amperes  over  a  copper  wire  1000  ft.  long  with  5 
volts  applied  at  its  terminals,  that  is,  5  volts  drop,  what  must  be  the  cross  section  of 
the  wire?     (Aid:   apply  Ohm's  Law  to  find  the  resistance  required  and  then  pro- 
ceed as  in  Example  B.}     Ans.    21,000  circular  mils. 

D.  What  pressure  is  required  to  force  a  current  of  50  amperes  over  a  copper  wire 
1600  ft.  long  which  has  a  cross  section  of  20,000  circular  mils?     Ans.   42  volts. 

E.  Make  a  table  of  the  specific  resistances  of  the  materials  represented  in  the 
table  of  relative  conductivities  in  Article  91. 

F.  If  two  copper  wires  of  equal  length  have  resistances  of  4  and  9  ohms,  respec- 
tively, and  the  diameter  of  the  first  is  |  inches,  what  is  the  diameter  of  the  other? 
Ans.   T*2  inches. 

G.  If  the  resistance  of  a  coil  of  wire  is  found  to  be  105  ohms,  and  a  piece  of  the 
same  wire,  which  is  10  ft.  long,  has  a  resistance  of  1.5  ohms,  how  many  feet  of  wire 
are  contained  in  the  coil?     Ans.    700  ft. 


92  ELECTRICITY   AND   MAGNETISM 

102.    Circuits  in  Series.  —  When  the  current  passes  around  its  circuit 
in  a  single  path,  the  path  is  termed  a  Series  Circuit.     The  path  may  be 

made  up   of  different   mate- 

x >v      rials  which  are  of  various  di- 

I  ^ — ^  mensions,   but   the  resistance 

.    —  g«/  &~r—         of  the  whole   is   the  sum   of 

the  resistances  of  all  the 
parts.  Thus,  suppose  we 
have  a  circuit  like  that  shown 
in  Figure  45,  where  A  is  a 
battery  of  large  cells  having 
a  resistance  of  .5  of  an  ohm, 

B  is  a  small  incandescent  lamp  having  a  carbon  filament  of  5  ohms 
resistance,  and  C  and  D  are  connecting  wires  having  resistances  of  .1 
and  .2  ohms  respectively.  The  total  resistance  of  the  circuit  is  the 
sum  of  these,  or  .5  +  5  -f  .1  -f-  .2  =  5.8  ohms. 

The  same  condition  exists  when  water  flows  through  pipes.     Thus, 
suppose  in  Figure  46  that  A,  B,  and  C  are  three  pipes  of  different 


FlG.  45.  —  Illustration  of  a  Series  Circuit  com- 
posed of  a  Battery,  Conducting  Wires,  and 
an  Incandescent  Lamp. 


PUMP 
f 

fc 
| 

,/i~ 

~.  . 
\ 
^ 

} 

} 

X 

h         A                             B                        C         ^ 

—           |  I  ril  VA!  VF 

TANK 

FlG.  46.  —  Hydraulic  Analogue  of  Series  Circuit. 


sizes  connected  in  series  for  drawing  water  from  a  tank.  Evidently  the 
separate  resistances  to  the  flow  of  the  water  introduced  by  these  differ- 
ent pipes  must  be  added  together  to  get  the  total  frictional  resistance 
from  the  tank  to  the  valve.  In  the  illustration,  the  tanks  and  pump 
also  form  parts  of  the  circuit,  and,  therefore,  if  the  total  resistance  of 
the  circuit  is  desired,  the  resistances  of  these  parts  must  be  added  to 
those  of  the  pipes. 


ELECTRIC  CIRCUITS  AND   THE   FLOW  OF  ELECTRICITY       93 


PROBLEMS 

A.  What  pressure  will  be  required  to  force  2  amperes  through  a  series  circuit 
containing  a  dynamo  armature  of  |  an  ohm,  conducting  wires  of  i\  ohms,  and  a 
lamp  filament  of  100  ohms  resistance  ?     Ans.    204  volts. 

B.  What  pressure  must  be  supplied  to  a  line  of  copper  wire  which  is  400  ft.  long 
and  has  a  cross  section  of  100,000  circular  mils,  in  order  that  200  amperes  may  be 
caused  to  pass  through  it  in  series  with  an  electrolytic  vat  which  has  an  apparent 
resistance  of  .03  ohms?     Ans.    14.4  volts. 

C.  A  dynamo  supplies  no  volts  to  a  copper  wire  circuit  400  ft.  long  which  has 
a  cross  section  of  2100  circular  mils.     This  circuit  supplies  a  lamp  which  calls  for 
^current  of  2  amperes.     What  is  the  hot  resistance  of  the  lamp  filament?     Ans. 

ohms. 

/D.  Ten  9  ampere  arc  lamps,  each  requiring  45  volts  pressure,  are  connected  in 
series  by  a  copper  wire  having  a  total  length  of  5000  ft.  and  a  cross  section  of  10,000 
circular  mils.  The  circuit  also  contains  a  dynamo  armature  of  5  ohms  and  a  dynamo 
magnet  coil  of  3  ohms  resistance.  What  is  the  total  pressure  required  to  keep 
9  amperes  flowing  through  the  circuit?  Ans.  569^  volts. 

103.  Circuits  in  Parallel.  —  If  two  wires  are  connected  in  parallel 
(that  is,  so  that  a  current  divides  between  them,  as  shown  in  Fig.  47), 
the  current  flowing  in  each  is  equal  to  the  pressure  between  their  common 
terminals  divided  by  their  individual  resistances.  For  instance,  if  the 

4  OHMS 


6  OHMS 


'. 12  VOLTS >• 

FIG.  47. —  Branched  or  Parallel  Circuit. 

two  wires  have  resistances  of  4  and  6  ohms  respectively,  and  the  press- 
ure between  their  terminals  (the  points  A  and  B,  Fig.  47)  is  12 
volts,  the  current  flowing  through  the  first  wire  is  *£-  =  3  amperes,  and 
that  through  the  second  is  -^  =  2  amperes. 

We  have   already   seen  that  the  current  which  flows  through  any 


94  ELECTRICITY  AND    MAGNETISM 

resistance  on  account  of  a  fixed  pressure  is  inversely  proportional  to  the 
resistance.  This  is  shown  by  Ohm's  Law.  Accordingly,  the  currents 
flowing  through  the  two  wires  of  the  previous  example  should  be  in  the 
proportion  of  -J  and  -J-.  This  is  true,  since  3  is  J  of  12  and  2  is  ^  of  12. 
The  total  current  flowing  through  the  circuit  containing  the  two  wires 
in  parallel  is  evidently  2  plus  3,  or  5  amperes.  Since  the  pressure  caus- 
ing these  5  amperes  to  flow  through  the  wires  is  1 2  volts,  the  resistance 
of  the  circuit  between  A  and  B,  or  the  Joint  Resistance  of  the  two 
wires  in  parallel,  must  be  ±g-9  or  2.4  ohms.  This  may  be  conveniently 


FIG.  48.  —  Hydraulic  Analogue  of  Branched  Circuit. 

calculated  directly  from  the  conductivities,  which,  it  will  be  remembered, 
are  reciprocal  or  inverse  to  the  resistances.1  The  conductivity  of  the 
first  wire  is  therefore  \  and  that  of  the  second  is  J. 

The  joint  capacity  of  two  or  more  pipes  which  deliver  water  between 
two  tanks  is  equal  to  the  capacities  of  all  the  separate  pipes  added 
together.  Thus  suppose  in  Figure  48,  A,  B  and  C  are  three  pipes  con- 
necting the  two  tanks.  Evidently  more  water  will  flow  through  two 
pipes  in  a  given  time  than  through  one  alone,  and  still  more  will  flow 
through  three  pipes  ;  hence  as  pipes  are  added  between  the  tanks  the 
resistance  to  the  flow  of  water  is  decreased,  that  is  to  say,  the  conduc- 
tivity is  increased.  The  capacities  of  the  pipes  for  carrying  water,  that 
is  their  conductivities,  must  therefore  be  added  together  to  get  the  total 
conductivity  for  the  flow  of  water  from  the  higher  tank  to  the  lower  one. 

1  Article  23. 


ELECTRIC   CIRCUITS   AND   THE   FLOW   OF   ELECTRICITY        95 

We  also  say  what  means  exactly  the  same  thing,  though  it  is  put  in 
different  words,  when  we  say  that  the  reciprocals  of  the  individual  pipe 
resistances  must  be  added  to  give  the  joint  conductivity  of  all  together. 
As  this  sum  gives  the  combined  conductivity  of  all  the  pipes,  its  recipro- 
cal will  be  the  combined  resistance. 

In  the  same  way  the  joint  conducting  power  of  electric  circuits  which 
are  connected  in  parallel,  or  Divided  Circuits  as  they  are  often  called,  is 
equal  to  the  conducting  powers  of  the  parts  added  together.  The  joint- 
conducting  power  or  conductivity  in  the  previous  example  is  therefore  \ 
plus  £,  or  -f%.  The  resistance  of  the  divided  circuit  is  the  inverse  of  this, 
which  is  equal  to  -1-2-,  or  2.4,  as  previously  calculated. 

This  shows  that  simply  adding  together  the  resistances  of  the  indi- 
vidual parts  of  a  circuit  will  not  always  give  the  total  resistance  of  the 
circuit.  In  fact,  such  an  addition  gives  the  total  resistance  only  when 
all  the  individual  resistances  belong  to  parts  of  the  circuit  which  are 
connected  in  series. 

A  little  consideration  of  what  precedes  will  show  that  when  two  wires 
of  equal  resistance  are  connected  in  parallel,  their  joint  resistance  \sjust 
half  as  great  as  the  resistance  of  either  wire.  If  three  wires  of  equal 
resistance  are  connected  in  parallel,  their  joint  resistance  is  just  one- 
third  as  great  as  the  resistance  of  one  of  the  conductors,  and  so  on.  If 
the  wires  of  equal  resistance  were  connected  in  series  instead  of  parallel, 
the  resistances  would  be  two,  three,  and  so  on,  times  as  great  as  a  single 
wire.  A  simple  rule  for  calculating  the  joint  resistance  when  only  two 
wires  are  connected  in  parallel,  is  to  multiply  together  the  individual 
resistances  of  the  wires  and  divide  this  product  by  the  sum  of  the  indi- 
vidual resistances.  This  comes  directly  from  the  laws  of  the  electric 
current  and  the  resistances  of  divided  circuits,  as  explained  above. 
But  it  is  generally  simpler  where  there  are  more  than  two  branches  in 
parallel,  to  consider  the  conductivities  when  calculating  the  joint  resist- 
ance of  parallel  circuits,  in  the  way  that  has  also  been  explained  above. 

PROBLEMS 

A.  Suppose  an  electric  battery  is  connected  to  an  external  circuit  of  two  parallel 
branches,  one  of  these  having  a  resistance  of  20  ohms,  and  the  other  a  resistance  of 
40  ohms,  what  proportion  of  the  total  current  flows  through  each  branch?  Ans.  f 
and  l. 


96  ELECTRICITY  AND   MAGNETISM 

B.  What  is  the  resistance  of  a  series  circuit  made  up  of  the  following  resistances : 
1st  part,  4  ohms;   2d  part,  2  ohms;   3d  part,  \\  ohms;  and  what  would  be  the  joint 
resistance  if  the  parts  were  joined  in  parallel?     Ans.    yi-  ohms,  and  f  ohms. 

C.  Four  parallel  circuits  of  1,2,  4,  and  5  ohms  resistance,  respectively,  have  20 
volts  impressed  upon  their  terminals.    What  is  the  total  current  that  flows?    Ans.    39 
amperes.     How  much  current  flows  through  each  branch?     Ans.  20,  10,  5,  4. 

D.  What  is  the  joint  conductivity  of  three  parallel  branches  which  have  respec- 
tively 4,  5,  and  20  ohms  resistance?     Ans.    1.     What  is  the  joint  resistance?     Ans. 
2  ohms. 

E.  What  is  the  joint  resistance  of  four  parallel  branches  which  have  respectively 
I,  4,  5,  and  20  ohms  resistance?     Ans.   f  ohms. 

F.  If  the  resistance  of  a  wire  is  4  ohms,  what  must  be  the  resistance  of  another, 
which  when  put  in  parallel  with  it  makes  the  joint  resistance  3  ohms?  Ans.  12  ohms. 

G.  The  joint  parallel  resistance  of  five  wires,  each  of  the  same  resistance,  is  5 
ohms,  what  is  the  resistance  of  each  of  the  wires?  Ans.  25  ohms. 

H.  What  is  the  joint  resistance  of  four  wires  in  parallel  which  have  resistances, 
respectively,  of  \,\,  J,  and  ^  of  an  ohm?  Ans.  ^  ohms. 

/.  What  is  the  joint  resistance  of  three  circuits  in  parallel  which  have  respectively 
resistances  of  I,  .5  (=  |  ),  .2  (—  ^)  ohms?  Ans.  .125  (=  -£•)  ohms. 

J.  If  ten  similar  incandescent  lamps,  connected  in  parallel  at  an  electrolier,  have 
a  joint  resistance  of  20  ohms,  what  is  the  resistance  of  each  lamp?  Ans.  200  ohms. 

K.  Three  copper  circuits  in  parallel  supply  a  building  with  300  amperes  to  run 
electric  lamps.  The  wires  composing  the  circuits  are,  respectively,  of  75,000,  105,000, 
and  120,000  circular  mils  cross  section  and  1000  feet  long.  If  the  satisfactory  oper- 
ation of  the  lamps  requires  100  volts  at  their  terminals,  how  many  volts  must  be 
impressed  upon  the  wires?  (Aid:  Add  the  circular  mils  together  and  compute  the 
joint  resistance;  then  find  the  volts  lost  in  the  wires.)  Ans.  110.5  v°lts. 

L.  There  are  four  incandescent  lamps  of  different  sizes,  placed  in  parallel  upon  a 
circuit.  These  have  respective  resistances  of  100,  150,  200,  and  300  ohms.  What 
total  current  passes  through  this  group  of  lamps,  when  100  volts  is  applied  at  its  ter- 
minals? Ans.  2.5  amperes. 

M.  A  copper  wire  with  a  cross  section  of  105,000  circular  mils,  600  ft.  in  length, 
was  used  for  transmitting  a  current  of  100  amperes  between  two  buildings.  The 
current  was  afterward  increased  to  150  amperes  and  a  second  wire  added  in  parallel 
with  the  first  of  such  size  that  the  drop  of  pressure  in  the  circuit  was  the  same  as 
before.  Of  what  cross  section  was  the  second  wire?  (Aid:  The  total  wire  cross 
section  must  be  increased  in  proportion  to  the  increase  of  current.)  Ans.  52,500. 

N.  Ten  battery  cells  of  2  volts  pressure  and  i  ohm  internal  resistance  each 
are  connected  in  parallel  and  short  circuited  by  a  wire  of  negligible  resistance.  What 
is  the  current  that  flows  through  the  short  circuit  wire?  Ans.  20  amperes. 

O.  Four  insulated  telegraph  wires  have  a  common  terminal  connected  to  one 
terminal  of  a  20  volt  battery.  The  other  terminal  of  the  battery  is  connected  to  the 


ELECTRIC  CIRCUITS  AND  THE  FLOW  OF  ELECTRICITY        97 


earth.  If  the  insulating  resistance  of  the  wires  from  the  earth  are  respectively 
100,000,  200,000,  250,000,  and  1,000,000  ohms,  how  many  thousandths  of  .an  ampere 
of  leakage  current  will  flow  between  the  wires  and  earth?  Ans.  .36  milliamperes. 

P.  A  bare  transmission  line  is  supported  by  500  glass  insulators  each  of  which  has 
on  a  certain  day  an  insulation  resistance  of  5,000,000  ohms.  What  is  the  insulation 
resistance  of  the  line?  Ans.  10,000  ohms. 

Q.  A  certain  telegraph  wire,  50  miles  long,  has  an  insulation  resistance  to  earth 
of  40,000  ohms.  How  much  current  (in  thousandths  of  amperes)  will  leak  to  earth 
when  a  battery  of  20,  i£  volt  battery  cells  are  connected  in  series  between  the  line 
and  the  earth?  Ans.  .6  milliamperes. 

104.  Series  and  Parallel  Circuits  Combined.  —  Circuits  are  sometimes 
spoken  of  as  Simple  Circuits  when  the  parts  are  all  in  series,  and 
Branched,  Compound,  or  Derived  Circuits,  when  the  parts  are  in  parallel. 
Parallel  connection  is  sometimes  called  connection  in  Multiple  or 
Multiple  Arc. 

The  total  resistance  of  a  circuit  made  up  of  parts  connected  in  series  is 
equal  to  the  sum  of  the  individual  resistances  of  all  the  parts}  The  total 
resistance  of  a  circuit  made  up  of 
parts  connected  in  parallel  is  equal 
to  the  reciprocal  of  the  total  con- 
ductivity of  the  circuit,  and  the  total 
conductivity  is  equal  to  the  sum  of 
the  individual  conductivities  of  the 
parts? 

When  part  of  the  total  circuit  is 
made  up  of  conductors  in  parallel 
it  is  necessary  to  first  calculate  the 
joint  resistance  of  that  part  and 
then  add  that  to  the  resistance  of 
the  remainder  of  the  circuit  which 
is  in  series  with  the  branched  por- 
tion. It  is  easily  seen  that  the 
joint  resistance  of  conductors  in 
parallel  is  equal  to  the  resistance 
of  a  single  conductor  with  which 


6  OHMS 


5  OHMS 


7^  OHMS 
FIG.  49.  —  Compound  Circuit. 


1  Article  102. 


2  Article  103. 


98 


ELECTRICITY  AND   MAGNETISM 


they  might  be  replaced  without  changing  the  total  resistance  of  the  circuit. 

A  circuit  which  contains  a  portion  composed  of  two  conductors  in 

parallel  is  shown  in  Fig- 
ure 49.  Suppose  that  the 
resistances  in  ohms  of 
the  different  parts  are  as 
marked,  then  the  total 
resistance  of  the  circuit  is 
1 2  ohms.  If  the  pressure 
developed  by  each  of  the 
two  cells  which  are  repre- 
sented by  the  usual  sign, 


B 


ft! 


is  1.2  volts,  the  current 
flowing  through  the  circuit 

2.4 
is  --i=.2  amperes. 

12 

This  would  be  analogous 
to  a  system  of  water  pip- 
ing between  two  tanks  as 
seen  in  Figure  50.  In 

this  case  the  resistance  of  B  and  C,  formed  by  taking  the  reciprocal  of 
the  sum  of  their  conductivities,  must  be  added  to  the  resistance  of 
A  to  get  the  total  pipe  resistance  between  the  tanks. 


FIG.  50.  —  Hydraulic  Analogue  of  the  Compound  Cir- 
cuit illustrated  in  Fig.  49  (omitting  the  pump,  which 
in  the  analogy  takes  the  place  of  the  electric  bat- 
tery). 


PROBLEMS 

A.  A  wire  of  2  ohms  resistance  is  connected  in  series  with  a  group  of  three  wires 
in  parallel  which  are  of  4,  5,  and  20  ohms  resistance,  respectively.     What  is  the  total 
resistance?     Ans.   4  ohms. 

B.  A  copper  wire  200  ft.   long  and  10,500  circular  mils  in  cross  section  is  in 
series  with  two  parallel  wires  400  ft.  long.     One  of  the  latter  wires  has  a  cross 
section  of  7000  circular  mils,  and  the  other  a  cross  section  of  14,000  circular  mils. 
What  is  the  resistance  of  the  circuit  ?     Ans.    .4  ohms. 

C.  A  group  of  wires  in  parallel,  of  2  and  6  ohms  resistance,  respectively,  is  con- 
nected in  series  with  another  group  of  three  wires  in  parallel,  with  I,  3,  and  6  ohms 
resistance,  respectively.     If  the  conductor  by  means  of  which  the  two  groups  are 
connected  in  series  has  a  resistance  of  1.5  ohms,  what  is  the  total  resistance  of  the 
system?     Ans.    3|  ohms. 


ELECTRIC  CIRCUITS  AND  THE  FLOW  OF   ELECTRICITY        99 

D.  Four  cells  which  give  a  pressure  of  1.5  volts  and  have  an  internal  resistance 
of  2  ohms,  each,  are  connected  in  parallel.     The  cells  supply  current  to  a  circuit  of 
|  an  ohm  resistance.     What  current  flows?     Ans.    \\  ampere. 

E.  If  the  battery  of  Example  D  is  connected  in  two  parallel  sets  of  two  cells  in 
series,  what  current  flows?     Ans.    \\  amperes. 

F.  We  have  six  battery  cells  of  2  volts  pressure  and  2  ohms  internal  resistance 
each.     How  shall  they  be  connected  to  get  the  greatest  current  through  a  3  ohm  cir- 
cuit ?      {Aid :    In  order  that  the  largest  C2irrent  may  be  caused  to  flow  through  a 
circuit  by  a  given  number  of  battery  cells  ;  or  a  given  current  may  be  caused  to  flow 
by   the  smallest  mimber   of  cells ;    the   cells  must  be  grouped  so  that  the  internal 
resistance  of  the  battery  is  as  nearly  as  possible  equal  to  the  external  resistance  in 
the  circuit.      Therefore,  group  the  cells  so  that  the  external  resistance  is  as  nearly  as 
possible  eq^^al  to  j  ohtns.}     Ans.  Two  parallel  sets  of  three  cells  in  series. 

G.  A  building  has  ten  electric  fire-alarm  bells,  each  of  10  ohms  resistance,  con- 
nected in  parallel.     If  these  bells  act  as  ordinary  resistances  and  the  connecting  wires 
are  of  negligible  resistance,  what  is  the  least  number  of  2-volt  battery  cells,  having  an 
internal  resistance  of  2  ohms,  that  would  be  required  to  send  .2  of  an  ampere  through 
each  bell  ?      (Aid :   Read  aid  to  Example  F,  and  group  cells  so  that  internal  and 
external  resistances  are  as  nearly  as  possible  equal.)      Ans.    8;   4  parallel  sets  of 
2  cells  in  series. 

H.  The  conductors  to  a  group  of  25  incandescent  lamps  are  of  .2  ohms  resistance. 
Each  lamp  has  a  hot  resistance  of  200  ohms  and  requires  .5  of  an  ampere  for  its  oper- 
ation. What  is  the  total  pressure  required  ?  Ans.  102.5  vo^s. 

/.  A  dynamo  which  delivers  500  amperes  at  a  pressure  of  20  volts  is  to  be  used 
for  supplying  current  to  20  electrolytic  vats,  each  of  which  requires  100  amperes  at 
a  pressure  of  5  volts.  How  shall  the  vats  be  connected  up  ?  Ans.  5  parallel  sets 
of  4  vats  in  series. 

/.  A  building  contains  one  hundred  100  volt  incandescent  lamps  of  200  ohms  hot 
resistance  each,  connected  in  parallel.  The  copper  wires  from  the  dynamo  to  the 
building  are  400  feet  long  (total).  What  size  must  the  conducting  wire  be  if  the 
dynamo  delivers  its  current  at  a  pressure  of  no  volts?  (Aid:  10  volts  are  left  for 
forcing  the  current  through  the  wire.)  Ans.  21,000  circular  mils. 

K.  400  incandescent  lamps  in  parallel,  of  \  amperes  and  100  volts  each,  are  to  be 
operated  by  a  storage  battery.  The  battery  cells  have  an  open  circuit  pressure 
of  2  volts  and  an  internal  resistance  of  .0037  of  an  ohm  each.  How  many  cells 
will  be  required  if  each  cell  is  not  to  discharge  over  40  amperes?  Ans.  5  parallel 
rows  of  54  cells  in  series. 

L.  Suppose  an  electric  battery  of  20  volts  open  circuit  pressure  and  6|  ohms 
internal  resistance  is  connected  to  two  external  branches  which  are  in  parallel,  one 
of  these  having  a  resistance  of  20  ohms  and  the  other  a  resistance  of  40  ohms;  how 
much  current  flows  through  the  battery  and  how  much  through  each  of  the  external 
circuits?  Ans.  I,  f,  and  \  amperes. 


100  ELECTRICITY   AND   MAGNETISM 

105.  Shunts.  — When  one  wire  is  connected  in  parallel  with  another 
it  is  often  called  a  Shunt,  because  it  switches  off  or  Shunts  a  part  of  the 
current  from  the  other  wire.     The  wire  to  which  a  shunt  is  attached  is 
said  to  be  Shunted.     Special  shunts  put  up  in  boxes  are  frequently  used  to 
protect  electrical  instruments  which  are  required  for  electrical  measure- 
ments, their  purpose  being  to  shunt  a  known  part  of  the  current  around 
the  instruments  when  the  currents  are  so  great  that  the  instruments 
might  be  injured  if  the  total  current  passed  through  them. 

106.  Fall  of  Pressure  along  a  Circuit.  —  Since  Ohm's  Law  shows  that 
the  electrical  pressure  between  two  points  in  a  circuit  is  equal  to  the 
current  flowing  in  the  circuit  multiplied  by  the  resistance  of  the  part  of 
the  circuit  between  the  points,  we  may  say  that  the  pressure  along  a 
wire  falls  or  "  drops  "  in  proportion  to  the  resistance  passed  over.    Thus, 
suppose  the  terminals  of  a  copper  wire  of  uniform  cross  section  and  10 
feet  long  are  connected  to  the  poles  of  an  electric  battery  furnishing 
a  pressure  of  two  volts.     Now,  since  equal  lengths  of  the  uniform  wire 
may  be  considered  as  having  equal  resistances  and  all  parts  of  the  wire 
carry  the  same  current,  the  electrical  pressure  measured  between  the 
middle  of  the  wire  and  one  end  must  be  equal  to  the  pressure  measured 
between  the  middle  and  the  other  end,  and  this  must  also  be  equal  to 
one  volt  or  one-half  the  total  pressure  measured  between  the  ends  of  the 
wire.     In  the  same  way  the  pressure  measured  across  (that  is,  the  "  drop 
of  pressure  "  in)  any  portion  of  the  wire  bears  the  same  proportion  to  the 
two  volts'  total  pressure  as  the  length  of  the  portion  bears  to  the  whole 
length  of  the  wire. 

If  one  end  of  the  wire,  while  still  connected  to  the  battery,  is  con- 
nected to  the  ground  (by  attaching  to  a  water  or  gas  pipe)  it  may  be  con- 
sidered as  being  a  zero  pressure ;  then  the  other  end  of  the  wire  is  at  an 
actual  pressure  of  two  volts.  (The  difference  of  pressure  between  the 
two  ends  of  the  wire  was  considered  before  without  taking  into  account 
their  actual  pressures.  The  same  thing  is  often  done  in  considering  the 
flow  of  water  or  gas  through  a  pipe.)  The  middle  of  the  wire  is  now  at 
a  pressure  of  one  volt,  while  2\  feet  (or  one-quarter  the  length  of  the 
wire)  from  the  higher  end  the  pressure  is  i  \  volts,  and  7^  feet  (or  three- 
quarters  of  the  length  of  the  wire)  from  the  upper  end  the  pressure  is  \ 
volt  (see  Fig.  51): 


ELECTRIC  CIRCUITS  AND  THE  FLOT&\OF 


FIG.  51. —  Illustration  of  Fall  of  Pressure  along  a  Wire  of 
Uniform  Resistance. 


If  the  wire  were  not  of  a  uniform  cross  section,  or  were  composed 
in  different  parts  of  different  metals,  the  resistances  of  equal  lengths 
would  no  longer  be  the 
same.  The  pressures 
measured  across  the 
portions  of  the  wire 
would  no  longer  be 
directly  proportional 
to  the  lengths  of  the 
portions,  but  would  be 
proportional  to  their 
resistances,  as  before. 

107.  General  Law  for  Fall  of  Potential  in  a  Circuit.  —  This  general 
rule  may  therefore  be  written  as  a  result  of  Ohm's  Law  :  the  electrical 
pressure  along  a  conductor  through  which  a  given  current  flows,  falls 
directly  as  the  resistance  passed  over.  The  same  rule  holds  in  the  case 
of  gas  or  water  flowing  through  a  pipe.  Suppose  it  requires  10  pounds 
pressure  to  cause  500  gallons  of  water  to  flow  per  minute  through  a 
certain  straight  pipe  200  feet  long.  If  the  pipe  is  cut  in  half,  5 
pounds  pressure  is  sufficient  to  pass  the  same  amount  of  water 
through  either  half.  If  pressure  gauges  are  attached  with  proper 
precautions  to  the  pipe  at  intervals  of  20  feet,  each  gauge  will  show 
a  pressure  of  one  pound  less  than  the  preceding  one,  when  taken  in  the 
direction  of  the  current.  This  shows  that  the  pressure  falls  directly  as 
the  resistance  passed  over  as  in  the  case  of  the  electric  current. 


PROBLEMS 

A.  A  copper  wire  of  uniform  size  100  feet  long  and   of  10  ohms  resistance  is 
connected  to   the   terminals  of  a  20  volt  battery  of  negligible  internal  resistance. 
If  the  negative  terminal  of  the  battery  is  considered  to  be  at  zero  potential,  what  is 
the  potential  of  the  wire  at  each  successive  10  foot  mark,  measured  from  the  nega- 
tive terminal?     Ans.    2,  4,6,  8,  10,  12,  etc.,  volts. 

B.  Three  wires  of  2,  6,  and  8  ohms  resistances  are  connected  in  series  between  the 
terminals  of  a  battery  which  gives  a  terminal  pressure  of  2  volts  when  so  connected. 
What  is  the  pressure  between  either  battery  terminal  and  the  wire  joints?    Ans.   .25 
or  1.75,  and  I  or  I  volts. 


102  ELECTRIC  JTY  AND   MAGNETISM 

C.  A  battery  cell  gives  1.5  volts  on  open  circuit,  and  has  an  internal  resistance  of 
3  ohms.    To  what  will  the  terminal  pressure  of  the  cell  fall  on  account  of  the  drop  of 
pressure  in  the  cell  itself,  when  a  current  of  .1  ampere  flows?     Ans.  To  1.2  volts. 

D.  Ten   40  volt,  10   ampere  lamps   are    connected  in  series,  with  1000  feet  of 
copper  wire  between  each  pair  of  lamps.     The  wire  has  a  cross  section  of  10,500 
circular  mils.     If  the  outside  terminal  of  the  first  lamp  is  at  a  pressure  of  5  volts, 
what  is  the  pressure  at  the  farther  terminal  of  each  lamp?     Ans.  55,  105,  155,  etc., 
volts. 

E.  A  magnet  coil  of  2  ohms  resistance  is  supplied  with  current  through  a  wire  of 
I  ohm  resistance.     The  current  is  supplied  by  a  battery  of  ten  battery  cells  in  series, 
each  of  which  has  an  internal  resistance  of  .2  of  an  ohm  and  gives  2  volts  pressure 
on  open  circuit.     What  is  the  pressure  between  the  terminals  of  the  coil?     (Aid: 
The  required  pressure  is  -f  of  20  volts.)     Ans.    8  volts. 

F.  It  is  desired  to  shunt  the  1000  ohm  coil  of  a  galvanometer  so  that  when  it  is 
connected  into  a  circuit  only  \  of  the  current  in  the  circuit  will  pass  through  this  coil. 
What  is  the  resistance  of  the  shunt?     Ans.    500  ohms. 

G.  If  a  uniform  wire  20  feet  long  measures  I  ohm,  what  is  the  fall  of  pressure 
per  foot  when  I  ampere  flows  through  it,  and  also  when  2  amperes  flow  through  it? 
Ans.        a 


QUESTIONS 

29.  What  is  a  mil? 

30.  What  is  a  circular  mil? 

31.  How  many  circular  mils  are  there  in  a  round  wire  50  mils  in  diameter? 

32.  What  must  be  the  diameter  of  a  wire,  in  mils,  in  order  that  it  may  have  a 
cross  section  of  400  circular  mils? 

33.  What  is  a  mil  foot? 

34.  What  is  specific  resistance? 

35.  What  is  the  numerical  value  of  the  specific  resistance  of  copper? 

36.  Is  the  specific  resistance  of  iron  greater  or  less  than  that  of  copper? 

37.  How  may  the  resistance  of  a  copper  wire  be  calculated? 

38.  W7hat  is  a  series  circuit? 

39.  Give  an  example  of  a  series  circuit. 

40.  How  is  the  total  resistance  of  a  series  circuit,  made  up  of  a  number  of  parts, 
determined? 

41.  What  are  circuits  in  parallel? 

42.  What  is  joint  resistance? 

43.  What  are  divided  circuits? 

44.  How  may  the  total  current  in  a  divided  circuit  be  found  if  the  pressure  and 
individual  resistances  are  known? 

45.  How  may  the  joint  resistance  of  two  parallel  wires  be  found  if  the  individual 
resistances  are  known? 

46.  What  is  the  reciprocal  of  the  resistance  of  a  wire? 


ELECTRIC  CIRCUITS   AND  THE   FLOW   OF   ELECTRICITY      103 

47.  Will  adding  the  individual  resistances  of  wires  in  parallel  give  the  joint  resist- 
ance? 

48.  What  will  adding  the  conductivities  of  wires  in  parallel  give? 

49.  What  will  the  reciprocal  of  the  combined  conductivity  of  wires. in  parallel 
give? 

50.  How  do  you  know  that  the  joint   resistance  of  five  wires  in  parallel,  each 
of  I  ohm  resistance,  is  one-fifth  of  an  ohm? 

51.  What  are   simple  circuits?     Compound  and   divided  circuits?     Multiple  or 
multiple  arc  circuits? 

52.  How  can  you  find  the  resistance  of  a  compound  circuit? 

53.  If  a  wire  of  £  ohm  is  connected  in  series  with  two  parallel  wires  each  having 
a  resistance  of  I  ohm,  what  is  the  total  resistance? 

54.  What  are  shunts? 

55.  How  does  the  potential  or  pressure  fall  along  a  uniform  wire? 

56.  What  does  the  fall  of  pressure  depend  upon  in  a  wire  made  up  of  several  pieces 
of  different  sizes  and  different  materials? 

57.  Give  a  water-pipe  analogy  to  electric  fall  of  potential. 


CHAPTER    VIII 

ELECTRICAL  ENERGY,  HEATING  EFFECTS  OF  ELECTRIC  CURRENTS, 
AND  MISCELLANEOUS  EFFECTS  OF  ELECTRIC  CURRENTS 

108.  Electric  Work  and  its  Unit  of  Measurement.  —  When  one  cou- 
lomb of  electricity  is  passed  through  a  wire  under  the  pressure  of  one 
volt,  a  certain  amount  of  work  is    done,    exactly   as   another  certain 
amount  of  work  is  done  when  a  gallon  of  water  is  raised  a  foot  in  height 
by  means  of  a  pump.     In  the  case  of  the  water  the  work  done  is  meas- 
ured in  Foot  Pounds.     A  foot  pound  means  an  amount  of  work  which  is 
done  when  a  force  equivalent  to  one  pound'1  s  weight  has  caused  a  body  to 
move  through  a  distance  of  one  foot.     If  one  lifts  a  pound  of  sugar  or 
other  material  through  a  vertical  distance  of  one  foot,  he  has  caused  the 
movement  by  the  exertion  of  a  force  equivalent  to  one  pound's  weight, 
and  therefore  has  done  one  foot  pound  of  work. 

In  order  to  determine  the  foot  pounds  of  work  done  in  pumping  water, 
the  pressure  under  which  the  water  is  pumped  must  be  converted  into  its 
equivalent  feet  of  Head  and  the  quantity  of  water  must  be  given  -trf  pounds. 
The  weight  of  a  gallon  of  water  is  about  8|  pounds.  Consequently,  if 
one  gallon  of  water  is  passed  through  a  pipe  under  a  pressure  which  is 
equivalent  to  one  foot  of  Head,  about  8J  foot  pounds  of  work  are  done 
(ix8i  =  8i). 

When  one  coulomb  of  electricity  is  passed  through  a  wire  under  a 
pressure  of  one  volt,  the  amount  of  work  done  is  called  one  Joule,  after 
the  name  of  Joule,  a  great  English  scientist  and  engineer. 

109.  Power  and  its  Unit  of  Measurement.  —  As  a  general  thing  we  do 
not  care  to  pump  a  single  gallon  of  water  through  a  pipe,  but  we  wish  to 
pump  a  given  number  of  gallons  per  minute.     In  this  case  for  each  gallon 
passed  per  minute  through  the  pipe  under  a  pressure  which  is  equivalent 
to  the  head  of  one  foot,  about  81  foot  pounds  of  work  must  be  done 

104 


WORK  AND  POWER  10$ 

every  minute.  Suppose  it  is  desired  to  pump  120  gallons  (1000 
pounds)  of  water  per  minute  through  a  pipe  under  ahead  of  33  feet,  the 
work  required  to  do  this  is  33,000  foot  pounds  per  minute. 

The  rate  at  which  work  is  done,  that  is,  the  amount  done  in  a  given 
time,  such  as  a  minute,  is  called  Power.  Mechanical  power  is  ordinarily 
divided  into  units  called  Horse  Power.  A  horse  power  is  equal  to  33,000 
foot-pounds  of  work  done  per  minufe,  so  that  in  the  last  example  exactly 
one  horse  power  is  required  to  move  the  water. 

The  horse-power  hour  is  frequently  used  as  a  unit  of  work.  It  is  the 
amount  of  work  done  by  a  horse  power  working  for  one  hour,  and  is 
equal  to  1,980,000  foot  pounds  (33,000  x  60  =  1,980,000). 

The  horse  power  of  a  waterfall  is  calculated  in  a  way  which  is  similar 
to  the  preceding  examples.  Suppose  a  stream  discharges  480  gallons, 
or  4000  pounds  of  water  per  minute,  over  a  fall  25  feet  high,  the  power 
of  the  water  is  100,000  foot  pounds  (100,000  =  25  x  4000)  per  minute, 
or  a  little  over  three  horse  power. 

The  horse  power  of  steam  engines  is  also  calculated  in  a  similar  man- 
ner. For  instance,  in  an  engine  which  is  supplied  with  steam  that 
exerts  an  average  pressure  on  the  piston  of  40  pounds  per  square  inch 
along  the  whole  stroke,  and  the  piston  of  which  has  a  surface  of  100 
square  inches,  the  total  pressure  exerted  by  the  steam  on  the  piston  is 
4000  pounds.  If  the  stroke  of  the  engine  is  i  foot,  the  piston  moves 
2  feet  per  revolution,  and  consequently  the  steam  exerts  8000  foot 
pounds  of  work  (8000  =  2  x  4000)  in  each  revolution.  If  the  engine 
runs  at  250  revolutions  per  minute,  the  v/ork  done  by  the  steam  is 
2,000,000  foot  pounds  (2,000,000  =  250  x  8000)  per  minute,  or  just  a 
little  more  than  60  horse  power.  This  is  called  the  indicated  horse 
power  of  the  engine.  Most  of  it  is  available  for  driving  machinery,  but 
a  portion  is  used  in  overcoming  the  friction  of  the  engine  itself. 

110.  The  Watt.  —  We  have  already  explained  in  this  chapter  that  when 
a  coulomb  of  electricity  is  sent  through  a  wire  under  a  pressure  of  one 
volt,  an  amount  of  work  is  done  which  is  called  a  joule ;  we  have  also 
explained  in  Article  22,  that  a  current  of  one  ampere  is  a  current  which 
conveys  one  coulomb  per  second.  Consequently  when  a  current  of  one 
ampere  is  passed  through  a  wire  under  a  pressure  of  one  volt,  the 
amount  of  work  done  is  equal  to  one  joule  per  second.  This  represents 


106  ELECTRICITY  AND   MAGNETISM 

a  certain  amount  of  power  which  is  called  a  Watt,  after  James  Watt,  a 
great  English  engineer,  and  inventor  of  the  modern  steam  engine. 
The  power  represented  by  one  watt  is  equal  to  one  seven  hundred  and 
forty-sixth  part  of  a  horse  power,  or  there  are  746  waffs  in  a  horse 
power.  In  speaking  of  the  power  of  electrical  machinery,  it  has  become 
customary  to  use  the  electrical  term  "  watt,"  and  for  a  larger  and 
frequently  more  convenient  unit  the  Kilowatt  is  used.  This  is  equal  to 
looowatts,  or  about  ij  horse  power. 

When  a  steady  electric  current  flows  through  a  circuit,  the  power  used 
in  the  circuit  is  equal  to  the  current  multiplied  by  the  total  pressure 
causing  the  current  to  flow ;  that  is,  the  power  in  watts  is  equal  to  the 
current  in  amperes  multiplied  by  the  pressure  in  volts,  or  P=  C  x  E. 
Part  of  this  power  may  be  used  in  causing  electrochemical  action  (by 
charging  a  storage  battery,  for  instance),  or  it  may  be  used  in  driving 
machinery  through  the  medium  of  an  electric  motor;  but  some  of  the 
power  is  always  used  in  overcoming  the  resistance  of  the  wires  which 
convey  the  current.  This  is  somewhat  similar  to  the  use  of  some  of  the 
indicated  power  of  a  steam  engine  in  overcoming  the  friction  of  the 
engine  itself. 

PROBLEMS 

A.  2000  coulombs  of  electricity  are  passed  through  an  electrolytic  vat  each  sec- 
ond, under  a  pressure  of  6  volts.     How  many  joules  of  work  are  expended  in  an 
hour?     Ans.    43,200,000. 

B.  If  3730  watts  are  expended  in  a  circuit,  how  many  horse  power  are  being 
developed?     Ans.    5. 

C.  If  10  horse  power  of  mechanical  energy  were  converted  into  electrical  energy 
how  many  watts  would  be  developed?     Ans.    7460. 

D.  100  horse  power  expended  continuously  for  one  hour  will  produce  how  many 
kilowatt  hours  (kilowatt  working  for  one  hour)  ?     Ans.    74.6  kilowatt  hours. 

E.  In  example  A  how  many  horse  power  are  being  used?     Ans.    16  (approx.). 

F.  How  many  foot  pounds  of  work  will  be  expended  in  a  minute  by  a  current 
of  373  amperes  flowing  under  a  pressure  of  20  volts?     Ans.    330,000. 

G.  A  25  horse  power  engine  drives  a  dynamo.     If  the  dynamo  gives  out  80  per 
cent  of  the  power  supplied  to  it,  what  number  of  kilowatts  does  it  develop  ?      Ans. 
14.92. 

H.  An  elevator  weighing  1000  pounds  is  to  be  lifted  at  the  rate  of  198  ft.  per 
minute.  If  the  driving  motor  delivers  75  per  cent  of  the  electrical  energy  it  receives, 
how  many  kilowatts  must  be  supplied  to  the  motor?  Ans.  5.968. 


CONSERVATION  OF  ENERGY  IO; 

/.  A  current  of  25  amperes  flows  through  a  circuit  under  a  pressure  of  100  volts, 
what  is  the  power?  Ans.  2500  watts. 

J.  If  100  watts  are  expended  in  a  circuit  by  a  current  of  5  amperes,  what  is  the 
pressure  required  to  drive  the  current  through  the  wire?  Ans.  20  volts. 

K.  If  100  incandescent  lamps,  using  100  volts  each,  are  connected  in  fifty 
parallel  sets  of  two  lamps  in  series,  and  if  they  use  a  total  of  5  kilowatts,  what  is  the 
current  used  by  each  lamp?  Ans.  .5  amperes. 

L.  A  set  of  silver  refining  vats  have  I  H.  P.  applied  to  them  at  an  average 
pressure  of  3.73  volts  per  vat.  How  much  silver  will  be  deposited  in  an  hour?  Ans. 
805  gr. 

111.  Conservation  of  Energy. — When  mechanical  power  is  used  in 
overcoming  friction  or  other  forms  of  resistance,  it  is  not  lost,  but  is  con- 
verted into  an  equivalent  amount  of  heat  which  is  another  form  of  energy. 
A  general  law  may  be  stated  thus  :  Energy  (that  is,  the  capability  of  doing 
work)  is  never  destroyed,  but  it  may  be  transformed  from  one  form  into 
another.     This  is  called  the  Law  of  the  Conservation  of  Energy. 

When  power  is  transformed  from  one  form  to  another,  there  is  always 
some  loss  of  the  amount  of  useful  power.  The  apparently  lost  power 
has  not  been  destroyed,  but  has  been  converted  into  heat.  For  instance, 
when  the  mechanical  power  conveyed  by  a  running  belt  is  changed  by 
means  of  a  dynamo  of  satisfactory  size  into  electrical  power,  about  ten 
per  cent  of  the  available  power  is  lost.  That  is,  the  electrical  power 
delivered  by  the  dynamo  is  about  ten  per  cent  less  than  the  mechanical 
power  which  is  given  to  the  dynamo.  This  difference  has  not  been  de- 
stroyed, but  has  been  converted  into  heat  in  overcoming  the  friction  of 
the  dynamo  bearings,  the  resistance  of  the  wire  windings  of  the  dynamo, 
and  in  other  ways.  A  dynamo  which  is  in  operation  is  always  found  to 
be  warmer  than  the  surrounding  air,  which  shows  that  some  of  the  power 
delivered  to  it  is  changed  into  heat  that  goes  to  warm  the  machine.  The 
usefulness  of  this  amount  of  power  is  therefore  lost,  but  the  energy  is 
not  destroyed. 

112.  Power  used  in  overcoming  Electrical  Resistance.  —  The  power 
which  is  used  in  overcoming  the  electrical  resistance  of  a  wire  when  a 
current  is  passed  through  it  is  converted  into  heat  which  warms  the  wire. 
The  heat  produced  is  proportional  to  the  number  of  watts  expended  in 
causing  the  electric  current  to  flow  through  the  resistance  of  the  wire ; 
and  this  is  equal  to  the  difference  of  pressure  at  the  terminals  of  the 


108  ELECTRICITY  AND   MAGNETISM 

wire  multiplied  by  the  current  flowing  in  it  (P=  C  x  E),  provided  all 
the  power  expended  in  that  part  of  the  circuit  is  used  in  heating  the 
wire. 

According  to  Ohm's   Law,  pressure  is  equal  to  current  times  resist- 


Consequently  C  times  E  is  equal  to  C  times  C  R,  or  C  squared  times 
R.  Hence  the  power  required  to  overcome  the  resistance  of  a  wire  is 
equal  to  the  square  of  the  current  multiplied  by  the  resistance  or 

P=  CE=  C*R. 

By  again  substituting  according  to  Ohm's  Law,  it  may  be  shown  that 
the  power  lost  in  a  wire  is  also  equal  to  the  pressure  squared  divided  by 
the  resistance,  or 

P-  CE-          - 

-&--& 

Power  is  in  every  case  given  in  watts,  provided  the  current  is  given  in 
amperes  and  the  resistance  is  given  in  ohms  or  the  pressure  in  volts. 

Since  the  portion  of  the  available  electrical  power  of  a  circuit  which  is 
lost  in  heating  the  conductors  is  equal  to  the  current  squared  times  the 
resistance  of  the  conductors,  it  is  often  spoken  of  as  the  C  squared  R 
loss. 

PROBLEMS 

A.  A  current  of  50  amperes  is  passed  through  a  resistance  of  5  ohms.  How  many 
watts  are  expended  in  heating  the  wire?     Ans.  12,500. 

B.  An  incandescent  lamp  requires  .6  of  an  ampere  of  current.    The  resistance  of 
its  filament  is  200  ohms.     How  many  watts  are  required  for  it?     Ans.    72. 

C.  A  wire  of  20  ohms  resistance  first  has  a  current  of  6  amperes  and  then  a  current 
of  18  amperes  passing  through  it.    How  many  times  greater  is  the  heating  effect  of  the 
latter  current  than  that  of  the  former?    Ans.    9. 

D.  A  copper  wire  10,500  circular  mils  in  cross  section  and  1050  feet  long  carries 
a  current  of  50  amperes.     What  must  be  the  cross  section  of  an  aluminum  wire  of  the 
same  length  to  carry  the  same  current  with  the  same  loss?     Ans.   19,090  cir.  mils. 

E.  It  is  found  that  2  kilowatts  are  wasted  upon  a  copper  wire  1000  feet  long 
when  20  volts  are  impressed  upon  its  terminals.     How  large  is  the  wire?     (First, 
find  resistance  of  the  wire  by  Ohm's  Law  and  then  the  cross  section.)     Ans.   52,500 
cir.  mils. 


THE  CALORIE  1 09 

F.  Current  is  supplied  to  a  group  of  50  incandescent  lamps,  connected  in  parallel, 
each  having  a  resistance  of  180  ohms,  through  a  wire  of  .4  of  an  ohm  resistance. 
What  per  cent  of  the  total  power  is  lost  in  the  wire?    (Aid :  Find  resistance  of  lamps; 
the  loss  will  be  proportional  to  resistance.)     Ans.    10  per  cent. 

G.  If  10  kilowatts  are  transmitted  over  a  wire  of  certain  resistance  and  at  a  pressure 
of  100  volts,  what  pressure  will  be  required  to  transmit  the  same  power  at  \  as  much 
loss  in  the  wire?     (Aid:  The  loss  in  a  wire  varies  inversely  as  the  square  of  the 
pressure.)     Ans.   200  volts. 

H.  If  it  requires  a  wire  of  500,000  circular  mils  cross  section  to  transmit  100 
kilowatts  at  a  pressure  of  100  volts  over  a  wire  15,000  ft.  long  at  a  required  loss,  how 
large  a  wire  will  be  required  to  transmit  the  same  power  at  1000  volts  pressure 
with  the  same  expenditure  of  energy  ?  (Aid :  The  cross  sections  of  the  wires  vary 
inversely  as  the  squares  of  the  pressures.)  Ans.  5000  cir.  mils. 

/.  How  far  can  100  kilowatts  be  transmitted  over  a  copper  wire  having  a  resistance 
of  .005  of  an  ohm  per  100  ft.  with  a  loss  of  10  kilowatts  in  the  wire,  at  100,  500, 
looo,  and  10,000  volts,  respectively  ?  (Aid  :  Find  the  current  and  then  the  loss  per  100 
ft.  Divide  the  loss  in  watts  per  100  ft.  into  10,000  (i.e.  10  kilowatts),  and  the  result 
will  be  the  number  of  hundreds  of  feet.)  Ans.  200,  5000,  20,000,  and  2,000,000  ft. 

J.  Two  Leclanche  cells  are  connected  in  series  in  opposition  to  a  third  one.  The 
pressure  of  each  cell  is  1.5  volts,  and  its  resistance  5  ohms.  If  the  free  terminals  of 
the  three  cells  are  connected  together  by  a  wire  of  negligible  resistance,  how  much 
power  is  being  expended  in  the  resistance  of  the  cells?  (Aid:  Take  the  difference 
of  the  two  opposing  pressures.)  Ans.  .15  watts. 

K.  If  ten  1.05  volt  Daniell  cells  are  connected  in  series  in  a  circuit  with  seven  1.5 
volt  Leclanche  cells  connected  in  opposition  to  them,  how  much  work  will  be  done 
in  the  circuit?  Ans.  None. 

L.  A  Leclanche  cell  has  at  a  certain  instant  an  internal  resistance  of  8  ohms,  a 
total  pressure  of  1.7  volts,  and  a  counter  pressure,  due  to  polarization,  of  .5  of  a  volt. 
The  cell  is  connected  to  an  external  circuit  of  8  ohms  resistance.  What  power  is  it 
giving  to  the  external  circuit?  Ans.  .045  watts. 

113.  The  Calorie  and  its  Relation  to  the  Joule.  —  It  is  possible  to 
measure  an  electric  current  by  the  heat  produced  when  it  is  passed 
through  a  known  resistance.  This  is  usually  done  in  an  instrument 
called  a  Calorimeter  (Fig.  52),  which  is  a  vessel  containing  water  or  some 
other  liquid  in  which  the  resistance  is  immersed.  The  vessel  usually  is 
double  walled  or  arranged  in  some  other  way  so  that  it  will  not  lose 
heat  rapidly  by  radiation  into  the  air.  A  thermometer  is  immersed 
in  the  liquid  to  determine  its  rise  of  temperature  due  to  the  heat  given 
it  from  the  wire.  The  amount  of  heat  which  is  required  to  raise  the 
temperature  of  a  gramme  of  water  one  degree  of  the  centigrade  scale 


no 


ELECTRICITY  AND   MAGNETISM 


WATER/  /-. /--4M 


is  called  a  Calorie.  The  number  of  calories  given  by  the  wire  to  the 
water  in  the  calorimeter  is  determined  from  the  amount  of  water 
and  its  rise  in  temperature,  proper  corrections  being  made  for  the 

effect  of  the  vessel. 

The  experiments  of  Joule, 
of  Rowland  (an  American 
scientist),  and  of  others 
have  shown  that  the  work 
represented  by  one  joule  is 
equivalent  to  the  heat  rep- 
resented by  practically  .24 
of  a  calorie.  Consequently, 
the  total  number  of  calories 
of  heat  produced  in  one  sec- 
ond by  the  current  passing 
through  the  wire  in  a  calo- 
rimeter is  equal  to  .24.  C~R. 
The  total  heat  produced  in 
any  time  is  also  equal  to 
.24  C^R  multiplied  by  the 
number  of  seconds  in  the 
time.  This  may  be  written 
in  the  form 

FIG.  52.  —  Simple  Form  of  Electric  Calorimeter.  H  =•  .2^C  RT. 

This  experimentally  determined  fact,  or  law  of  nature,  is  sometimes 
called  Joule's  Law. 

By  determining  the  total  heat  produced  in  the  calorimeter  in  a  fixed 
time,  when  the  current  is  passed  through  a  known  resistance,  the  value 
of  the  current  may  be  determined.  The  square  of  the  current  is  equal 
to  the  calories  divided  by  .24  times  the  resistance  multiplied  by  the 
time  in  seconds. 

It  may  be  seen  from  what  precedes  that  one  ampere  flowing  through 
a  resistance  of  one  ohm  expends  continuously  a  power  of  one  watt, 
which  is  equivalent  to  the  expenditure  of  one  joule  of  work  every  second, 
and  that  it  produces  .24  calories  of  heat  every  second. 


WIRE  RESISTANCE 


HEATING   EFFECT   OF  THE   CURRENT  III 

PROBLEMS 

A.  How  much  heat  will  be  developed  in  a  wire  having  a  resistance  of  .5  ohms  if  a 
current  of  10  amperes  flows  through  it  for  an  hour?     Ans.   43,200  calories. 

B.  If  a  pound  of  carbon  will  give  out  228,000  calories  in  its  combustion,  how  many 

joules  of  work  can  it  do  in  heating?   (Aid:  Joule  =  Ca  °ne  •  }    Ans.    950,000. 

V  -24      / 

C  If  a  battery  of  9  Daniell  cells  of  1.2  volts  pressure  and  2  ohms  internal  resist- 
ance each,  be  connected  up  in  three  sets  in  parallel  of  three  cells  in  series,  how  much 
heat  will  they,  in  one  hour,  develop  in  a  wire  of  2  ohms  resistance?  (Aid  :  Find  the 
total  pressure  and  resistance  of  the  battery  and  the  current  that  will  flow;  afterward 
apply  Joule's  Law.)  Ans.  1400  calories  (approx.). 

D.  Can  the  cells  in  Example  C  be  arranged  to  give  heat  to  the  wire  at  a  higher- 
rate?     Ans.   No. 

E.  How  many  joules  of  work  will   be  done  in  raising  the  temperature  of  600 
grammes  of  water  24  degrees  centigrade  ?     Ans.   60,000. 

F.  A  battery  is  connected  to  a  wire  coiled  up  in  a  calorimeter  which  contains 
600  grammes  of  water.     The  temperature  of  the  water  rises  10°  C.  in  10  minutes. 
If  \  of  the  heat  is  lost  by  radiation,  how  much  power  is  supplied  to  the  coil  by  the 
battery?     (Aid:  Watts  =  joules  expended  per  second.)     Ans.    50  watts. 

G.  A  current  was  passed  through  a  coil  of  wire  of  I  ohm  resistance  in  a  calorim- 
eter containing  400  grammes  of  water.     The  water  rose  8  centigrade  degrees  in 
temperature  in  400  seconds.     If  \  of  the  heat  was  lost  by  radiation,  how  much  cur- 
rent was  flowing  ?     (Aid :   Find  the  total  calories  ;  then  solve  for  C'mH =.24.  C^RT.} 
Ans.    6.4  amperes. 

H.  A  current  of  10  amperes  heated  the  water  of  a  calorimeter  5°  C.  in  20  minutes. 
An  unknown  current  heated  it  an  equal  amount  in  25  minutes.  If  the  difference  in 
radiation  was  negligible,  what  was  the  strength  of  the  second  current?  (Aid:  The 
rate  of  expenditure  of  energy  is  inversely  as  the  time.)  Ans.  8  amperes. 

/.  How  much  heat  will  an  incandescent  lamp  of  200  ohms  resistance  and  using 
.5  amperes,  give  off  per  hour?  Ans.  43,200  calories. 

J.  A  copper  wire  has  a  current  of  24  amperes  passing  through  it,  and  the  difference 
of  pressure  at  its  terminals  is  10  volts  when  its  temperature  is  75°  F.  How 
much  more  heat  will  be  produced  per  second  in  the  wire  when  carrying  the  same 
current  if  its  temperature  becomes  120°  F.?  (Aid:  10  per  cent  more  pressure  will  be 
used  in  the  second  case  as  the  resistance  of  the  wire  increases  I  per  cent  for  each  4.5° 
rise  of  temperature.)  Ans.  5.76  calories. 

K.  Five  horse  power  is  used  in  an  electrical  cook  stove.  How  much  heat  is  gener- 
ated per  minute?  (Aid:  One  calorie  =  .24  watts  for  one  second.)  Ans.  53,712 
calories. 

L.  If  the  heat  generated  by  the  current  in  a  wire  is  equal  to  the  power  required 
to  lift  300  pounds  n  feet  per  minute,  how  many  calories  are  expended  per  second? 
Ans.  17.9  (approx.). 


112  ELECTRICITY   AND   MAGNETISM 

114.  Temperature   of   Wire   carrying  Current.  —  The  actual  rise  of 
temperature  on  the  part  of  a  wire  when  a  current  passes  through  it  de- 
pends upon  several  things  in  addition  to  the  amount  of  heat  produced  in 
it.     A  long,  thick  wire  and  a  short,  thin  wire  of  the  same  material,  and 
having  the  same  resistance,  will  come  to  very  different  temperatures  when 
equal  currents  are  passed  through  them.    If  there  is  sufficient  difference 
in  their  diameters,  the  thin  wire  may  become  red-hot  on  account  of  the 
passage  of  a  current  which  is  only  sufficient  to  make  the  thick  wire 
appreciably  warm. 

When  a  current  passes  through  a  wire,  a  certain  amount  of  heat  is 
produced  during  every  second  that  the  current  flows.  For  a  short  time 
after  the  current  is  started,  the  wire  rises  in  temperature,  and  finally 
reaches  a  certain  fixed  temperature.  When  the  temperature  becomes 
fixed  it  is  evident  upon  a  little  thought  that  as  much  heat  must  leave  the 
wire  by  Radiation  to  surrounding  objects,  Convection  by  air  currents,  or 
Conduction  to  objects  touching  the  wire,  as  is  produced  by  the  flow  of 
the  current.  If  more  heat  is  given  to  the  wire  than  is  carried  off  by 
these  means,  its  temperature  must  rise ;  and  if  on  account  of  a  de- 
crease in  the  current  the  amount  of  heat  given  to  the  wire  is  less  for  a 
time  than  the  amount  given  off,  the  temperature  must  fall  until  the  two 
are  equal  again. 

The  capability  of  a  wire  to  get  rid  of  heat  by  radiation  and  convection 
depends  upon  the  color  and  condition  of  its  surface,  and  also  roughly 
upon  the  extent  of  the  surface.  The  amount  of  heat  which  leaves  any 
surface  in  a  second  also  depends  upon  the  number  of  degrees  by  which 
its  temperature  is  higher  than  that  of  the  air  and  surrounding  objects. 
The  amount  of  heat  which  is  required  to  bring  a  wire  to  a  given  temper- 
ature also  depends  upon  the  capacity  of  the  material  for  holding  heat,  or 
its  Specific  Heat,  as  it  is  called.  Consequently  the  actual  temperature 
to  which  any  wire  will  rise  when  carrying  a  certain  current  can  be  ex- 
actly determined  only  by  trying  the  experiment. 

115.  Effect  of  Insulating  Coverings.  —  The  fact  that  the  ability  of  a 
wire  to  emit  heat  is  directly  dependent  upon  the  extent  of  its  surface 
causes  a  wire  with  an  ordinary  insulating  covering  to  remain  cooler  in 
the  open  air  than  a  similar  wire  without  the  covering,  though  the  two 
wires  carry  equal  currents. 


PHYSIOLOGICAL   EFFECTS   OF  CURRENTS  113 

This  seems  at  first  sight  exactly  opposed  to  the  facts  as  seen  in 
covered  boiler  pipes.  There  is  no  contradiction,  however,  because  the 
thickness  of  the  insulation  is  entirely  comparable  with  the  diameter  of 
the  wire,  and  the  outside  surface  of  the  insulation  is  therefore  so  much 
greater  than  that  of  the  wire  that  the  additional  surface  more  than 
makes  up  for  the  difficulty  which  the  heat  experiences  in  getting 
through  the  insulation;  the  heat  thus  finds  it  easier  to  leave  the  wire 
which  has  the  insulation  on  it.  This  effect  is  most  decidedly  shown 
when  the  outer  surface  of  the  insulation  is  black. 

When  steam  pipes  are  covered  for  the  purpose  of  retaining  their  heat, 
the  thickness  of  the  covering  is  thin  compared  with  the  diameter  of  the 
pipes,  so  that  the  outside  surface  of  the  covered  pipes  is  not  much 
greater  than  the  surface  of  the  pipes  when  bare.  Consequently  the 
effect  of  the  thickness  of  the  covering  which  is  placed  in  the  path  of  the 
heat  as  it  leaves  the  pipe  is  greater  than  the  effect  of  the  increased  sur- 
face, and  the  heat  finds  it  more  difficult  to  leave  a  covered  pipe.  This 
is  especially  the  case  because  the  steam  pipes  are  covered  with  the  very 
best  heat  insulators. 

When  wires  are  closed  up  in  mouldings  or  placed  under  plaster,  as  is 
often  the  case  with  the  electric  light  wires  in  buildings,  they  become 
very  much  warmer  than  when  exposed  in  such  a  way  that  they  may  be 
cooled  by  air  currents. 

The  heating  effects  of  electric  currents  flowing  through  conductors 
are  depended  upon  for  the  operation  of  incandescent  lamps,  electric 
heaters,  and  some  other  devices  that  are  described  in  later  pages. 

116.  Physiological  Effects  of  Electric  Currents.  —  Electric  currents 
cause  various  effects  besides  those  of  electrochemistry,  electromag- 
netism,  and  electric  heating.  These  effects  are  of  many  kinds,  but  of 
small  commercial  importance,  and  in  most  cases  seem  to  be  due  to 
some  action  of  the  current  upon  the  molecules  of  the  material  through 
which  it  flows.  Some  of  the  effects  are  undoubtedly  due  to  electro- 
chemical action,  though  they  have  often  been  attributed  to  some  un- 
known action  of  the  current.  One  of  these  effects,  which  is  of  sufficient 
importance  outside  of  the  field  of  purely  speculative  science  to  require 
attention,  is  the  physiological  action  of  the  current.  Galvani  accidentally 
discovered  this  action  through  some  experiments  performed  upon  frogs. 
i 


114  ELECTRICITY  AND    MAGNETISM 

His  discovery  has  been  followed  up  by  many  scientists  down  to  the 
present  day,  and  a  vast  array  of  facts  has  been  determined  relating  to 
the  effects  of  currents  on  living  organisms. 

The  researches  of  these  scientists  have  shown  that  protoplasm,  which 
is  the  fundamental  basis  of  all  living  bodies,  has  the  power  of  contracting 
when  an  electric  current  passes  through  it.  Moreover,  a  living  animal 
nerve  is  always  excited  to  action  by  the  passage  through  it  of  an  electric 
current  from  an  external  source.  If  the  terminals  of  a  battery  cell  are 
touched  to  the  tongue,  a  peculiar  taste  may  be  noticed.  This  taste  may 
also  be  caused  by  laying  a  copper  and  a  silver  coin  upon  the  tongue 
with  their  edges  touching.  In  this  case  a  current  is  set  up  through  the 
metals,  the  saliva  of  the  mouth  serving  as  the  fluid.  If  the  terminals  of 
a  battery  cell  are  touched  to  the  temples,  or  placed  so  that  the  current 
flows  from  the  forehead  to  the  hand,  flashes  of  light  may  sometimes  be 
perceived,  due  to  the  excitation  of  the  nerves  of  the  eye  by  the  current. 
In  the  same  way  the  nerves  of  smell  and  hearing  may  be  excited. 

When  a  sufficiently  powerful  electric  current  is  passed  through  the 
ordinary  nerves,  a  feeling  of  tickling,  pricking,  or  pain  may  be  observed. 
If  the  current  is  sufficiently  strong,  it  may  cause  a  very  painful  muscular 
contraction,  and  if  excessive  the  current  may  cause  death.  The  muscu- 
lar and  nervous  effect  due  to  a  strong  current  is  ordinarily  called  a  Shock. 
The  severity  of  shock  depends  upon  the  amount  of  the  electrical  current 
which  flows  through  the  body,  but  it  also  depends  largely  upon  the 
physiological  condition  of  the  person  who  receives  the  shock. 

It  has  been  found  that  electric  currents  naturally  exist  in  the  living 
muscles  and  nerves  of  animals,  and  that  muscular  exertion  seems  to 
cause  them.  These  currents  disappear  with  the  death  of  the  animal, 
which  possibly  shows  that  the  electric  currents  have  some  function  in 
the  action  of  the  nervous  system.  The  capability  of  delivering  quite  a 
severe  electric  shock  to  marauders  exists  in  certain  animals  —  notably 
the  Gymnotus, — but  their  means  of  producing  the  electric  discharge  has 
not  been  disclosed  to  man. 

The  physiological  action  of  electric  currents  gives  a  good  basis  for 
their  use  in  the  treatment  of  certain  diseases,  and  they  have  been  used 
with  marked  success  in  some  cases.  The  electrolytic  effects  of  steady 
battery  currents  may  be  used  for  reducing  swellings,  hardening  tissues, 


THERMO-ELECTRIC  EFFECTS  115 

injecting  medicine  through  the  skin,  and  other  purposes.  Rapidly 
alternating  currents  and  high  pressure  static  discharges  have  proved  of 
service  on  account  of  their  effect  upon  the  nerve  system.  The  use  of 
a  cautery  blade  heated  by  electricity  has  become  quite  common.  The 
miniature  electric  light  in  combination  with  a  proper  system  of  mirrors 
has  made  it  possible  to  examine  many  of  the  internal  cavities  of  the 
body.  It  may  be  of  interest  to  add  that  a  great  proportion  of  the  elec- 
tromedical  appliances  that  are  commonly  advertised,  such  as  electric 
belts,  magnetic  brushes,  etc.,  are  not  only  absolutely  useless,  but  in  many 
cases  harmful.  Indeed,  electric  treatment  should  never  be  applied 
except  under  the  immediate  direction  of  a  trained  physician.  The  indis- 
criminate use  of  electrical  treatments  of  any  kind  is  likely  to  do  more 
harm  than  good. 

117.  Thermo-electric  Currents.  —  In  1821,  Seebeck,  a  Russian  by  birth 
and  German  by  education,  while  carrying  on  a  series  of  electric  experi- 
ments, under  the  inspiration  of  the  work  of  Oersted  (Article  119), 
found,  when  he  held  in  his  hand  one  of  the  junctions  of  a  circuit  com- 
posed of  antimony  and  copper  strips,  that  the  needle  of  the  galva- 
nometer in  circuit  was  deflected.  This  he  ascribed  to  the  heating  of  the 
junction,  since  if  he  held  his  hand  at  any  other  place  than  a  junction 
there  was  no  deflection.  He  also  found  that  cooling  one  of  the  junctions 
also  caused  a  deflection.  The  manifestations  of  this  phenomenon,  and 
those  allied  to  it,  are  called  Thermo-electric  Effects. 

It  has  been  found  that  electricity  may  be  generated  by  heating  or 
cooling  a  junction  in  a  circuit  composed  of  any  two  dissimilar  materials, 
such  as  two  metals,  two  liquids,  a  metal  and  a  liquid,  or  even  a  single 
material  which  has  slightly  different  physical  characteristics  in  its  parts. 
Thus,  for  instance,  a  copper  wire  may  be  annealed  in  part  of  its  length, 
when,  if  the  region  between  the  annealed  and  unannealed  part  is  heated, 
a  current  will  be  caused  to  flow  through  the  circuit. 

If  the  joints  in  the  circuit  of  unlike  metals  are  heated  equally,  no 
electromotive  force  is  set  up,  and  no  thermo-electric  current  flows.  A 
difference  in  the  temperatures  of  the  junctions  is  essential  to  the  exhibition 
of  the  effect. 

Thirteen  years  later  than  Seebeck's  discovery,  Peltier,  a  Paris  watch- 
maker, made  an  allied  discovery  —  that  an  electric  current,  when  it  flows 


ELECTRICITY   AND   MAGNETISM 


HUV 


across  the  junctions  of  unlike  metals  in  a  circuit,  may  either  heat  or  cool 
the  junction.  This  Peltier  Effect  is  the  reverse  of  the  thermo-electric 
effect  discovered  by  Seebeck.  Some  years  later,  Lenz  actually  suc- 
ceeded in  cooling  a  junction  by  the  Peltier  effect  to  so  low  a  tempera- 
ture as  to  cause  water  to  freeze. 

Lord  Kelvin,  somewhat  later,  made  additional  discoveries,  and  with 
others  added  to  the  sum  of  experimental  knowledge  regarding  thermo- 
electric effects,  but  the  cause  of  the  phenomena  has  never  been  dis- 
covered. 

118.  The  Thermopile.  —  Different  materials  set  up  very  different  ther- 
mo-electric pressures,  but  the  pressure  of  a  single  junction  of  any  pair  of 
metals  is  so  small  that  it  is  desirable  to  measure  it  in  microvolts  (mil- 

lionths  of  a  volt). 
Antimony  and  bis- 
muth furnish  the 
highest  pressure  of 
any  of  the  metals 
that  can  be  used 
satisfactorily.  Since 
the  pressure  which 
one  joint,  or  Couple 
as  it  is  called,  sets 
up  is  very  small,  a 
number  of  couples 
are  often  connected 
in  series.  When 
these  are  laid  up 
side  by  side,  so  that 
alternating  junctions  may  be  heated  and  cooled,  the  arrangement  is 
called  a  Thermopile.  Eigure  53  shows  a  thermopile  and  the  series  of 
couples  composing  it.  For  making  a  thermopile,  or  Thermo  Battery, 
as  it  is  also  called,  for  furnishing  comparatively  large  currents,  German 
silver  and  an  alloy  of  antimony  and  zinc  may  be  used  to  advantage, 
since  they  will  stand  a  high  temperature.  One  couple  of  such  a  battery 
will  give  about  .04  of  a  volt  when  the  hot  junctions  are  heated  to  as 
high  a  temperature  as  the  metals  will  stand  without  injury,  and  the  others 


e  View  of  a  Thermopile. 


THERMO-BATTERIES  1 1 7 

are  kept  at  the  temperature  of  the  air.  The  necessary  waste  of  heat 
which  occurs  in  using  thermo-batteries  makes  them  of  little  commercial 
value,  though  they  have  proved  of  great  service  in  some  lines  of  scien- 
tific investigation  on  account  of  the  steadiness  of  the  current  which 
they  produce,  and  the  convenience  attending  their  use. 

QUESTIONS 

1.  What  is  the  foot  pound? 

2.  What  is  the  joule? 

3.  Compare  work  done  by  water  flowing  under  pressure  to  electricity  flowing 
under  pressure. 

4.  If  50  coulombs  of  electricity  flow  under  a  pressure  of  10  volts,  how  many 
joules  of  work  will  be  done? 

5.  What  quantity  of  electricity  must  flow  under  a  pressure  of  2  volts  to  do  50 
joules  of  work? 

6.  If  20  coulombs  do  20  joules  of  work  in  flowing  through  a  wire,  what  is  the 
pressure? 

7.  What  is  power? 

8.  What  is  a  horse  power? 

9.  How  many  foot  pounds  of  work  are  done  by  a  horse  power  working  for  one 
minute? 

10.  How  may  the  power  of  a  waterfall  be  calculated  ?     Of  a  steam  engine  ? 

11.  What  is  the  unit  of  electric  power? 

12.  Who  was  the  watt  named  after? 

13.  How  many  watts  are  there  in  a  horse  power? 

14.  What  is  a  kilowatt? 

15.  How  many  watts  will  be  expended  if  a  current  of  10  amperes  flows  under  a 
pressure  of  10  volts? 

1 6.  Into  what  other  form  of  energy  is  electrical  energy  converted  by  the  passage 
of  a  current  through  a  resistance  ? 

17.  What  is  the  law  of  conservation  of  energy? 

1 8.  What  finally  becomes  of  the  energy  put  into  a  ball  when  it  is  thrown  into  the  air? 

19.  What  is  the  mechanical  power  transmitted  by  the  foot  to  a  sewing  machine 
finally  converted  into? 

20.  A  battery  cell  has  its  terminals  connected  by  a  wire.    What  two  transformations 
does  the  chemical  energy  of  the  cell  undergo? 

21.  A  coal  pile  feeds  a  boiler,  the  boiler  supplies  steam  to  an  engine,  the  engine 
drives  a  dynamo,  the  dynamo  drives  a  motor,  and  the  motor  runs  a  sawmill.     Name 
the  various  transformations  passed  through  by  the  energy  originally  in  the  coal. 

22.  There  are  losses  of  the  power  available  for  useful  purposes,  in  each  of  the 
transformations  of  question  21.     What  becomes  of  the  lost  power? 

23.  Is  the  power  lost  for  useful  purposes  in  any  transformation  ever  destroyed? 


Il8  ELECTRICITY   AND   MAGNETISM 

24.  Can  any  kind  of  a  machine  be  run  without  some  of  the  energy  that  is  given 
to  it  being  converted  into  a  useless  form  of  heat  energy? 

25.  Is  a  perpetual  motion  machine  possible?     (A  perpetual  motion  machine  is 
one  that  requires  no  energy  to  keep  it  going.) 

26.  What  becomes  of  the  energy  used  in  sending  a  current  through  a  wire? 

27.  What  is  the  power  in  watts,  lost  in  the  resistance  of  a  wire,  equal  to  in  terms 
of  the  resistance  and  current? 

28.  What  is  the  power  in  watts,  lost  in  the  resistance  of  a  wire,  equal  to  in  terms 
of  the  resistance  and  pressure? 

29.  What  is  the  pressure  in  volts,  required  in  sending  a  current  through  a  wire, 
equal  to  in  terms  of  the  resistance  and  current? 

30.  If  the  same  current  passes  through  three  wires  of  equal  length,  but  having 
cross  sections  in  the  proportion  of  one,  two,  and  three,  what  will  be  the  relative  losses 
by  heating  in  the  wires? 

31.  What  is  a  calorimeter? 

32.  Why  is  a  calorimeter  double  walled? 

33.  What  is  a  calorie? 

34.  What  is  Joule's  Law? 

35.  What  is  the  amount  of  heat  in  calories  that  will  be  expended  in  a  wire,  in 
terms  of  its  resistance,  the  current,  and  time? 

36.  How  can  a  calorimeter  be  used  as  a  current-measuring  instrument? 

37.  How  much  heat  will  2  amperes  produce  per  second,  in  calories,  in  flowing 
through  a  wire  with  a  resistance  of  I  ohm? 

38.  What  effect  has  the  form  of  a  wire  upon  its  temperature  when  a  current  is 
flowing  through  it? 

39.  What  are  radiation,  conduction,  and  convection,  in  reference  to  heat? 

40.  What  is  specific  heat? 

41.  If  a  current  is  passing  through  a  wire,  why  will  the  wire  rise  to  a  certain  tem- 
perature and  not  continue  indefinitely  to  grow  hotter? 

42.  What  are  the  elements  upon  which  the  dissipation  of  heat  from  a  wire  depend? 

43.  If  wire  is  closely  wrapped  with  insulating  material,  will  it  become  hotter  under 
the  influence  of  a  given  current  than  it  would  if  bare?     Why? 

44.  Compare  the  dissipation  of  heat  from  insulated  wires  and  covered  steam  pipes. 

45.  Will  a  wire  placed  within  a  moulding  carry  as  large  a  current  without  over- 
heating as  will  be  the  case  if  it  is  exposed  to  the  air? 

46.  Will  heat  be  dissipated  more  rapidly  into  the  air  from  a  wire  at  a  temperature 
of  200°  than  it  will  when  the  wire  is  at  ioo°? 

47.  What  are  some  of  the  physiological  effects  of  the  electric  current? 

48.  For  what  purposes  is  electricity  used  in  medicine? 

49.  What  is  thermo-electricity? 

50.  When  were  the  phenomena  of  thermo-electricity  first  noticed?     By  whom? 

51.  What  is  the  Peltier  effect? 

52.  What  is  a  thermopile? 


CHAPTER   IX 
ELECTROMAGN  ETISM 

119.  Historical. — The  real  connection  which  exists   between  mag- 
netism and  currents  of  electricity  was  not  made  generally  known  until 
Oersted,  a  Danish  scientist,  published  the  fact  in  1819  that  a  magnetic 
needle  is  disturbed  by  the  presence  of  an  electric  current  in  its  neigh- 
borhood.    This  fact  had  really  been  discovered  earlier,  but  it  did  not 
become  generally  known,  and  its  importance  had  not  been  recognized. 
It  had  also  been  known  that  under  some  conditions  lightning  discharges 
had  magnetized  steel  needles,  but  the  conditions  had  not  been  success- 
fully reproduced  by  experimenters.     The  publication  of  the  results  of 
his  experiments,  by  Oersted,  led  a  number  of  scientists  to  turn  their 
attention  during  the  early  part  of  the  nineteenth  century  to  a  determina- 
tion, as  complete  as  was  then  possible,  of  the  exact  relation  existing  be- 
tween electricity  and  magnetism.     We  are,  therefore,  entirely  justified  in 
crediting  Oersted  with  the  original  discovery  of  the  magnetic  effect  of 
the  electric  current — one  of  the  epoch-making  discoveries  of  the  world. 

120.  Effect  of  a   Current  flowing  near   a   Magnetic  Needle.  —  If  a 
magnetic  needle  is  placed  above  or  below  a  wire  which  carries  an  elec- 
tric current,  the  needle  will  turn  on  its  pivot  so  as  to  set  itself  as  nearly  as 
possible  at  right  angles  to  the  wire.      This    may  be   readily  tried   by 
connecting  a  short  piece  of  copper  wire  to  one  or  two  cells  of  a  gravity 
battery  and  holding  the  wire  above  the  needle  (Fig.  54)  while  the  cur- 
rent flows  through  it.     The  effect  on  the  needle  may  be  made  most 
evident  by  making  and  breaking  the  electric  circuit,  which  will  cause 
the  needle  to  swing  back  and  forth,  since  it  will  be  deflected  every  time 
the  circuit  is  closed  and  will  return  toward  its  position  of  natural  rest 
when  the  circuit  is  broken.     The  current  in  the  wire  has  the  greatest 
effect  in  causing  the  needle  to  deflect  from  the  north  and  south  position 
when  the  wire  also  lies  in  a  north  and  south  direction  —  that  is,  when 
the  wire  is  parallel  with  the  needle. 

119 


I2O 


ELECTRICITY   AND    MAGNETISM 


The  force  of  the  earth's  magnetism  is  what  causes  the  needle  to  turn 
into  a  north  and  south  position  and  stay  there  when  it  is  undisturbed  by 
other  magnetic  effects.  When  the  electric  current  is  placed  so  as  to 
flow  near  the  magnetic  needle,  the  needle  is  affected  by  the  force  of  an- 


FIG.  54.  —  Wire  held  above  Magnetic  Needle. 

other  magnetic  field  which  is  set  up  by  the  current,  and  which  tends  to 
make  the  needle  set  itself  at  right  angles  to  the  wire  carrying  the  cur- 
rent. The  needle  takes  an  intermediate  position  where  the  effect  of 
the  two  magnetic  forces  (that  due  to  the  earth  and  that  due  to  the  cur- 
rent) balance.  Its  new  position,  therefore,  depends  upon  the  magnitude 
of  the  force  due  to  the  magnetism  set  up  by  the  current  as  compared 
with  the  force  of  the  earth's  magnetism. 

121.  Direction  and  Strength  of  an  Electromagnetic  Field.  —  Mag- 
netism set  up  by  an  electric  current  is  called  Electromagnet  ism ;  but 
this  term  is  merely  a  convenient  indication  of  the  immediate  source  of 
the  magnetic  force,  since  the  magnetic  force  produced  by  a  permanent 
magnet  such  as  a  magnetized  piece  of  steel  or  the  earth  and  that  pro- 
duced by  an  electric  current  are  exactly  alike.  The  direction  of  the 
magnetic  force  due  to  electromagnetism  is  always  at  right  angles  to  the 
direction  of  the  current  which  produces  the  magnetism,  and  the  lines  of 
force  in  the  magnetic  field  due  to  the  current  in  a  cylindrical  wire 
must,  therefore,  be  circles  surrounding  the  wire.  The  strength  of  the 
magnetic  field  at  any  point  due  to  an  electric  current  near  by  depends  di- 
rectly upon  the  strength  of  the  current  and  inversely  upon  the  average 
distance  of  the  current  from  the  point.  The  reason  why  a  magnetic  field 
is  set  up  by  an  electric  current  is  entirely  unknown ;  merely  the  experi- 
mental fact  and  its  applications  are  known. 


ELECTROMAGNETISM 


121 


The  magnetic  field  which  surrounds  a  current  may  be  graphically 
shown  in  a  way  similar  to  that  used  to 
show  a  field  around  a  magnet.1  A  stout 
copper  wire  may  be  passed  vertically 
through  a  hole  in  a  horizontal  sheet  of 
stiff  paper.  If  iron  filings  are  sprinkled 
upon  the  paper,  they  will  arrange  them- 
selves in  circles  around  the  wire  when  a 
current  is  passed  through  it.  (Fig.  55.) 
If  a  small  magnetic  needle  or  compass 
is  placed  on  the  paper  with  its  centre 
over  a  line  of  filings,  the  needle  will 
tend  to  stand  at  a  tangent  to  the  line 
(Fig.  56).  An  independent  magnet  pole 
(if  such  a  thing  were  physically  possible) 
would  tend  to  move  continuously  around 
the  wire  along  any  one  of  the  lines  upon 


FIG.  .55.  —  Picture  of  Iron  Filings 
showing  the  Distribution  of 
Magnetism  around  a  Wire  car- 
rying an  Electric  Current.  The 
black  dot  represents  a  cross  sec- 
tion of  the  wire. 

which  it  might  be  placed. 


ERIDIAN  OF 

»• 

EARTH'S  MAGNETISM 


FlG.  56.  —  Direction  of  Magnetic  Lines  of  Force  which  surround  an  Electric  Current 
shown  by  Small  Compass  Needles,  NS. 

1  Articles  80  and  84. 


122 


ELECTRICITY  AND   MAGNETISM 


The  direction  in  which  the  magnetic  needle  points  when  near  a  wire 
carrying  an  electric  current  depends  upon  which  side  of  the  wire  it 
stands,  and  upon  the  direction  in  which  the  current  flows  in  the  wire. 
In  Figure  56  it  is  evident  from  the  position  of  the  magnetic  needles,  the 
black  ends  of  which  represent  north  poles,  that  the  positive  direction 
along  the  lines  of  force  is  there  left-handed,  or  opposite  to  the  direction 
of  motion  of  the  hands  of  a  clock.  If  the  direction  of  the  current 
were  reversed,  the  magnetic  needles  would  also  reverse  their  directions, 
showing  that  the  positive  direction  of  the  lines  of  force  has  a  fixed 
relation  to  the  direction  of  the  current. 

122.  Rules  for  determining  the  Direction  of  a  Field  around  a  Current. 
—  There  are  various  ways  of  remembering  the  relation  between  the  posi- 
tive direction  of  the  lines  of  force  and  the  di- 
rection of  the  current  which  produces  them. 
One  is  to  consider  an  ordinary  right-handed 
screw  which  is  being  screwed  into  or  out  of 
a  block  (Fig.  57).  If  an  electric  current  is 
considered  as  flowing  through  the  screw  in  the 
direction  which  the  screw  moves  through  the 
block,  then  the  positive  direction  of  the  lines  of 
force  is  shown  by  the  direction  in  which  the 
screw  turns.  Instead  of  a  screw  and  nut,  a 
corkscrew  being  screwed  into  or  out  of  a 

cork  may  be  thought  of.     The  rule  may  be 
FIG.  57.  — Illustration  by  Screw  y 

and  Nut  of  the  Relation    applied  to  Figure  54  by  way  of  illustration. 

Another  way  of  remembering  this  relation 
is  according  to  a  rule  proposed  by  Ampere, 
after  whom  the  unit  of  electric  current  was 
named.  Suppose  a  man  lying  in  the  wire  with  his  head  down  the  electric 
stream  (swimming  with  the  electric  current);  then  if  he  faces  a  magnetic 
needle  placed  near  the  wire,  the  north  pole  of  the  needle  will  tend  to  turn 
toward  his  left  hand. 

This  rule  may  also  be  applied  by  way  of  illustration  to  Figure  54.  The 
man  must  be  supposed  to  be  lying  flat  on  the  wire  with  his  face  toward 
the  magnetic  needle  and  his  head  pointing  toward  the  right-hand  edge 
of  the  page,  since  he  is  swimming  with  the  current.  His  right  hand  is 


between  the  Directions  of 
Current  and  the  Magnet- 
ism produced  by  it. 


ELECTROMAGNETISM 


123 


then  toward  the  reader  and  his  left  hand  away  from  the  reader.  The 
curved  arrows  show  that  the  north  pole  of  the  needle  tends  to  turn 
toward  his  left  hand. 

This  relation  between  the  direction  of  the  current  flow  and  the  deflec- 
tion of  a  magnetic  needle  gives  a  ready  method  for  determining  the 
direction  of  the  current  in  a  wire,  the  only  indi- 
cator which  is  required  being  a  small  compass. 
The  compass  may  be  placed  under  the  wire  and 
the  direction  toward  which  its  north  pole  turns 
noted.  Then  an  application  of  one  of  the 
rules  gives  the  direction  of  the  current.  This 
means  is  very  commonly  used  in  electrical 
manufacturing  establishments  and  in  testing 
laboratories. 

Another  rule  for  illustrating  the  relation  be- 
tween the  direction  of  the  current  in  a  wire 
and  the  direction  of  its  lines  of  force  is  this  : 
Grasp  the  wire  with  the  right  hand,  the  thumb 
being  extended  along  the  wire,  and  the  fingers 
being  wrapped  around  the  wire  ;  then  the  fingers 
point  in  the  positive  direction  along  the  lines 
of  force  when  the  thumb  points  in  the  direction 
of  flow  of  the  current. 

123.  Mutual  Force  acting  between  a  Magnet 
and  Current.  —  Since  we  know  that  a  force 
acting  between  two  bodies  always  affects  them 
both,1  we  may  expect  that  a  wire  which  carries 
a  current  will  tend  to  move  when  brought  near 
a  fixed  magnet.  This  may  be  readily  shown 
by  suspending  a  very  flexible  conducting  wire 

near  a  fixed  magnet  (Fig.  58).  When  a  current  is  passed  through  the 
wire  it  will  wind  itself  around  the  magnet.  If  the  current  is  reversed, 
the  wire  will  unwind  and  then  wind  around  the  magnet  again,  but  in  the 
opposite  direction. 

The  motions  of  the  wire  and  the  magnet  are  due  to  the  apparent 

1  Article  84. 


FIG.  58.  —  Illustration  of  the 
Effect  of  Magnet  on  Flexi- 
ble Wire  carrying  Electric 
Current. 


I24 


ELECTRICITY   AND    MAGNETISM 


FIG.  59. —  Illustration  of  Magnetic  Needle  placed  at  Centre  of  a 
Coil  of  Wire. 


tendency  of  magnetic  lines  of  force  to  move  out  of  a  position  where  they 
are  not  parallel  with  each  other  and  into  a  position  where  they  are 
parallel  with  each  other  and  in  the  same  direction^ 

124.  Solenoids.  —  By  applying  Ampere's  rule,  we  see  that  if  a  wire 
carrying  a  current  is  passed  above  a  magnetic  needle,  and  then  is 

turned    back    and 

X"  =z^\  passed  below  the 

needle,  both  the 
top  and  the  bottom 
branches  tend  to 
deflect  the  needle 
in  the  same  direc- 
tion, so  that  the  ef- 
fect on  the  needle 
is  increased.  (See 
Fig.  59.)  If  the 
two  branches  are 

equally  near  the  needle,  they  act  upon  it  with  equal  force,  and  thus  the 
total  force  on  the  needle  is  doubled.  By  coiling  the  wire  about  the  posi- 
tion of  the  needle,  each  additional  turn  will  cause  an  additional  deflect- 
ing force.  In  this  way  the  magnetic  effect  of  a  current  may  be  greatly 
multiplied.  It  has  already  been  said 
that  the  magnetic  force  at  a  point 
due  to  a  current  near  it  depends 
upon. the  strength  of  the  current.2 
We  now  see  that  when  a  current  is 
coiled  around  a  point  the  force  de- 
pends upon  the  strength  of  the  cur- 
rent  multiplied  by  the  number  of 
turns  in  the  coil.  This  product 

of  the  current  by  the  turns  is  usually  called  "  current  turns  "  or  "  ampere 
turns." 

When  a  wire  carrying  a  current  is  coiled  into  a  ring  or  helix,  the  lines 
of  force  which  surround  each  turn  seem  to  join  together  so  that  they 
belong  to  the  coil  or  winding  as  a  whole  (Fig.  60).  Such  coils  are  often 

1  Article  86.  2  Article  121. 


FIG.  60.  —  Picture  of  Loose  Solenoid  with 
its  Lines  of  Force. 


ELECTROM  AGNETISM  1 2  5 

called  Solenoids.  Such  coils,  when  a  current  is  passed  through  them, 
exhibit  all  the  magnetic  effects  which  are  shown  by  steel  magnets.  They 
attract  and  repel  magnets  and  other  solenoids,  and  attract  pieces  of  iron. 
If  suspended  so  that  they  are  free  to  swing,  they  turn  into  a  north  and 
south  position  exactly  like  magnets.  Figure  61  shows  an  iron  filing 
illustration  of  the  magnetic  field  within  a  solenoid.  The  illustration  is 
the  longitudinal  section  taken  along  the  axis  of  the  solenoid,  and  the 
black  dots  represent  the  indi- 
vidual conductors  of  the  coil. 

By  applying  either  of  the 
rules  for  finding  the  relation 
between  the  direction  of  a  cur- 
rent and  its  magnetic  field,  the 
polarity  of  a  solenoid  may  be 
readily  found.  This  relation  FlG-  6l«- picture  of  Iron  Filin^s  showins  the 

Distribution  of  Magnetism  inside  and  out- 

is  also  expressed  as  follows  :  side  of  a  Solenoid. 

If  a  person  face  one  end  of  a 

solenoid,  and  the  current  is  flowing  in  a  direction  counter  to  the  direc- 
tion of  the  hands  of  a  clock,  the  north  pole  of  the  solenoid  will  be  nearer 
him.  If  the  current  flows  clock-wise,  the  south  pole  will  be  nearer  him. 
This  magnetic  effect  of  coils  or  solenoids  led  Ampere  to  suppose  that 
all  magnetism  is  caused  by  electric  currents.  He,  therefore,  suggested 
that  the  molecules  of  magnetic  materials,  and  possibly  of  all  materials, 
have  little  electric  currents  flowing  around  them  which  make  them  into 
magnets.  This  is  called  "Ampere's  theory"  of  magnetism.1  If  the 
theory  is  correct,  it  gives  a  ready  explanation  of  why  magnetism  is 
found  in  various  materials,  but  it  still  leaves  unexplained  the  reason  for 
the  existence  of  the  electric  currents  in  the  molecules,  and  also  why  the 
electric  current  causes  magnetism.  Ampere's  theory  and  theories  of 
magnetism  advanced  by  other  scientists  have  been  before  the  scien- 
tific world  for  many  years,  but  their  correctness  has  not  yet  been  either 
proved  or  disproved.  We  must,  therefore,  rest  for  the  present  with  the 
evidence  that  the  molecules  of  magnetic  materials  appear  to  be  of  a 
magnetic  character,  while  those  of  non-magnetic  materials  appear  not  to 
be  magnetic. 

l  Article  81. 


126  ELECTRICITY  AND    MAGNETISM 

QUESTIONS 

1.  Who  discovered  the  magnetic  effect  of  the  electric  current?    At  what  date? 

2.  State  why  you  consider  Oersted's  discovery  of  great  value  to  the  world. 

3.  If  a  magnetic  needle  is  placed  near  a  wire  carrying  a  current,  what  position 
will  it  take? 

4.  If  a  magnetic  needle  lies  parallel  to  a  wire  placed  in  a  north  and  south  direc- 
tion, what  two  forces  act  upon  it  when  a  current  is  sent  through  the  wire? 

5.  Is  it  possible  to  measure  the  strength  of  a  current  by  its  magnetic  effect  upon 
a  needle? 

6.  Why  does  a  current  tend  to  deflect  a  needle? 

7.  What  is  electromagnetism? 

8.  Is  the  magnetism  created  by  an  electric  current  in  any  way  different  from  the 
magnetism  of  a  magnetized  steel  bar? 

9.  What  is  the  direction  of  the  magnetic  lines  with  respect  to  the  direction  of 
current  which  sets  them  up? 

10.  Upon  what  depends  the  strength  at  any  given  point  of  a  magnetic  field  which 
is  set  up  by  a  current? 

11.  How  much  stronger  is  the  magnetic  field,  set  up  by  a  current,  I  inch  from  a 
straight  wire  carrying  the  current,  than  it  is  2  inches  away? 

12.  If  one  wire  carries  three  times  as  much  current  as  another,  how  much  stronger 
will  the  magnetic  field  be  I  inch  from  the  former  than  it  is  one  inch  from  the  latter? 

13.  A  compass  is  placed  I  cm.  above  a  straight  wire  which  lies  north  and  south 
and  carries  a  current  of  I  ampere.     The  deflection  is  found  to  be  the  same  as  it 
was  when  the  compass  was  placed  two  centimeters  above  another  wire  lying  in  the 
same  direction.     How  much  current  was  flowing  in  the  second  wire? 

14.  How  can  it  be  shown  that  the  lines  of  force  about  a  cylindrical  wire  carry- 
ing a  current  are  in  the  form  of  circles? 

15.  Why  will  a  magnetic  needle  reverse  the  positions  of  its  poles  when  placed  on 
opposite  sides  of  a  current-bearing  wire? 

1 6.  Give  the  "  screw  rule  "  for  determining  the  direction  of  the  field  about  a  current. 

17.  Give  the  "  swimming  rule."     Give  the  thumb  and  hand  rule. 

1 8.  How  can  the  direction  of  a  current  be  determined  by  means  of  a  compass? 

19.  A  compass  needle  tends  to  stand  with  its  north  pole  to  the  east  when  placed 
under  a  wire  lying  in  a  north  and  south  direction.     Determine  the  direction  of  the 
current  in  the  wire  by  the  swimming  rule. 

20.  Answer  Question  19  by  means  of  the  screw  rule. 

21.  If  you  are  standing  beside  and  facing  a  wire  in  which  a  current  flows  from 
left  to  right,  and  you  place  a  compass  needle  over  the  wire,  will  the  north  pole  tend 
to  turn  toward  you  or  away  from  you? 

22.  While  standing  beside  and  facing  a  wire  you  place  a  compass  under  it  and 
find  that  the  north  pole  of  the  needle  moves  away  from  you.     Determine  whether  the 
current  is  from  left  to  right  or  vice  versa. 


ELECTROMAGNETISM 

23.  What  happens  to  a  flexible  wire  carrying  a  current  when  it  is  brought  near  a 
magnet? 

24.  Why  does  a  flexible  wire  carrying  a  current  tend  to  wind  about  a  magnet? 

25.  What  position  do  the  lines  of  force  of  two  fields  tend  to  take  when  the  fields 
are  brought  together? 

26.  Describe  two  experiments  that  show  that  the  current  flowing  in  a  wire  and  a 
magnet  both  tend  to  move  when  they  are  brought  near  each  other. 

27.  What  is  a  solenoid? 

28.  Why  will  a  needle  deflect  when  placed  within  a  turn  of  wire? 

29.  What  effect  on  the  magnetic  field  of  an  electric  current  is  produced  by  wind- 
ing the  wire  which  carries  the  current  into  a  solenoid  of  many  turns? 

30.  Upon  what  does  the  strength  of  field  in  a  solenoid  depend? 

31.  What  are  ampere  turns? 

32.  How  many  ampere  turns  has  a  coil  of  twenty  turns  which  carries  one-half  an 
ampere? 

33.  How  do  the  lines  of  force  arrange  themselves  in  and  about  a  solenoid? 

34.  In  what  ways  can  a  solenoid  be  compared  to  a  permanent  magnet? 

35.  What  led  Ampere  to  advance  his  theory  of  magnetism? 

36.  What  is  the  "  clock  "  rule  for  determining  the  relative  direction  of  current 
and  magnetism  in  a  solenoid? 

37.  If  the  north  pole  of  a  compass  needle  is  attracted  toward  the  end  of  a  sole- 
noid, before  which  you  are  standing,  what  is  the  direction  of  current  in  the  coil? 

38.  If  you  wish  to  have  a  south  pole  at  the  bottom  of  a  vertical  solenoid,  how 
must  the  current  be  made  to  flow? 

125.  Magnetizing  Effect  of  a  Solenoid  upon  Magnetic  Materials.  —  If 
a  bar  of  hard  steel  is  placed  in  a  solenoid  through  which  a  current 
is  passing,  it  becomes  strongly  magnetized,  and  it  remains  permanently 
magnetized  when  the  current  is  stopped  or  the  steel  is  withdrawn  from 
the  solenoid.  This  effect  is  exactly  the  same  as  would  be  obtained  by 
touching  the  steel  with  a  permanent  magnet,  but  the  magnetic  effect 
of  a  solenoid  with  many  turns  of  wire  may  be  made  much  greater  than 
that  of  any  permanent  magnet,  and  the  steel  may,  therefore,  be  more 
readily  saturated  by  the  solenoid. 

In  Article  89  it  is  said  that  a  magnetomotive  force  is  necessary  to 
maintain  magnetism  in  a  circuit.  Evidently  a  solenoid  creates  such 
a  difference  of  magnetic  potential  or  pressure,  or  it  could  not  maintain 
its  magnetism.  It  has  been  found  by  experiment  that  one  ampere  turn 
sets  up  nearly  one  and  one-quarter  units  of  magnetic  pressure.  Thus, 
if  we  had  a  solenoid  of  25  turns  with  a  current  flowing  through  it  of  4 


128 


ELECTRICITY  AND    MAGNETISM 


amperes,  nearly  125  units  of  magnetic  pressure  would  be  created. 
This  relation  may  be  expressed  in  the  following  formula,  for  conven- 
ience :  — 

M=  1.257  nc. 

where  M  is  the  magnetic  pressure,  and  nc  the  number  of  ampere  turns, 
n  being  the  turns  and  c  the  current.  The  exact  value  of  the  constant 

For  many  purposes  the  first  three 


(1.257)  is  equal  to  --or  1.2566  + 


significant   figures   give   sufficient   accuracy,  and  the  formula  is  then 
very  convenient. 

Now,  if  a  bar  of  soft  iron  is  placed  in  the  solenoid,  it  becomes  even 
more  strongly  magnetized  when  the  current  is  turned  on  than  did  the 
steel  bar.  When  the  current  is  turned  off,  the  soft  iron  loses  nearly  all  of 
its  magnetism.  If  the  bar  is  made  of  very  soft  Swedish  iron,  its  coercive 
force  is  so  small  that  the  least  tap,  after  the  current  is  shut  off,  shakes 
practically  all  of  the  magnetism  out  of  it.  Harder  and  less  pure  iron 
retains  a  little  of  the  magnetism  —  the  amount  depending  upon  the 
quality  of  the  iron.  The  magnetism  which  is  retained  by  iron  after  it 
has  been  magnetized,  and  the  magnetizing  influence  has  been  removed,  is 
called  Residual  Magnetism.  The  so-called  permanent  magnetism  of 

hard  steel  magnets  is  residual  magnetism 
very  firmly  retained  by  the  great  coercive 
force  of  the  hard  steel. 

126.  Electromagnets. — When  a  coil  of 
wire  is  wound  around  a  piece  of  soft  iron  or 
steel  for  the  purpose  of  getting  a  magnetic 
field,  the  combination  is  called  an  Electro- 
magnet. A  piece  of  hard  steel  might  be 
used  instead  of  soft  iron,  but  in  this  case  the 

\  "Ti  /  amount   of   magnetism  created  by  a  given 

V   L__,LII»         .^----i^k'Z  current  in   the    coil,   or  number  of  ampere 

turns,  would  be  much  less  than  is  produced 
in  soft  iron.  Soft  iron  or  steel  is  universally 
used,  therefore,  in  electromagnets. 

Electromagnets  are  of  the  greatest  value 
I  IG.  62.  —  Two  Horseshoe  Elec- 
tromagnets, in  the  electrical  industries  because  they  can 


ELECTROMAGNETISM  1 29 

be  built  of  practically  any  desired  size  and  form,  and  of  enormous  mag- 
netic strength.  The  magnets  of  commercial  dynamos  and  electric  motors 
are  always  electromagnets.  Figure  62  shows  two  forms  of  Horseshoe 
electromagnets. 

The  property  of  soft  iron  through  which  it  becomes  strongly  magnet- 
ized when  it  is  placed  within  a  solenoid,  and  then  loses  its  magnetism 
when  the  current  is  broken,  was  discovered  by  William  Sturgeon  of 
England  in  1825.  Very  shortly  after  Oersted's  discovery,  Sir  Humphry 
Davy,  Arago,  Ampere  and  others  had  magnetized  steel  needles  by  plac- 
ing them  in  solenoids,  but  it  was  reserved  to  Sturgeon,  an  otherwise  little 
known  scientist,  to  make  the  discovery  of  that  most  important  property 
of  soft  iron  —  the  dependence  of  its  magnetism  upon  the  continued 
presence  of  the  magnetizing  force  and  the  controllability  of  its  magnetism 
by  varying  the  magnetizing  force.  Like  Davy,  Faraday,  Henry,  and 
others  of  the  world's  great  discoverers  in  physical  science,  Sturgeon,  as 
a  boy,  was  an  artisan  apprentice,  and  gained  his  knowledge  of  science 
through  study  and  experiment  in  his  unemployed  hours. 

At  the  time  of  the  discovery  of  the  electromagnet  nothing  was  thought 
of  its  great  commercial  future  ;  but  it  was  welcomed  with  the  highest 
scientific  interest.  At  that  day  the  laws  of  electric  circuits  were  un- 
known, the  common  insulated  wire  of  to-day  was  not  made,  and  the 
manufacture  of  an  electromagnet  was  a  matter  of  much  labor.  More- 
over, the  only  sources  of  current  were,  at  first,  plain  zinc-copper  cells, 
and  later,  Grove,  Daniell,  or  similar  types  of  galvanic  cells.  Many  elec- 
tromagnets were  soon  made,  however,  and  their  effects  were  carefully 
studied  by  enthusiastic  scientists,  in  spite  of  the  difficulties  to  be  over- 
come. By  the  year  1845,  utt^e  more  tnan  a  na^  century  ago,  the 
investigators  had  succeeded  in  overcoming  their  lack  of  experimental 
facilities  and  had  mapped  out  the  laws  of  magnetic  circuits  very  much 
as  we  know  them  at  the  present  time.  Thus  was  laid  the  foundation  of 
the  profession  of  electrical  engineering. 

127.  Curve  of  Magnetization.  —  If  currents  of  different  strengths  are 
sent  through  the  coil  of  an  electromagnet,  the  strength  of  magnetism  pro- 
duced will  vary  with  the  current,  below  the  saturation  of  the  iron,  though 
not  in  direct  ratio ;  and  after  the  iron  is  saturated  very  little  additional 
magnetism  will  be  set  up  by  increasing  the  current.  Figure  63  shows  by 
K 


130 


ELECTRICITY  AND    MAGNETISM 


means  of  a  curve  the  typical  relation  between  ampere  turns  and  mag- 
netism. Distances  on  the  horizontal  line  are  proportional  to  the  ampere 
turns,  and  on  the  vertical  line  to  the  number  of  magnetic  lines  of  force 
per  square  centimeter  in  the  iron.  Such  a  curve  is  called  the  Curve  of 
Magnetization  of  the  electromagnet.  A,  in  the  figure,  represents  the 
region  in  which  the  saturation  of  the  iron  is  reached.  Beyond  this  re- 
gion, the  curve  shows  that  any  increase  of  ampere  turns  makes  only  a 
relatively  slight  increase  in  magnetism.  When  the  current  is  withdrawn, 


FIG.  63.  — Curve  of  Magnetization  of  Soft  Iron. 

the  coercive  force  will  retain  a  proportion  of  residual  magnetism  in  the 
iron,  and,  therefore,  if  the  current  is  removed  little  by  little,  a  second 
curve  will  be  made  as  shown  by  C  D,  in  Figure  64.  The  poorer,  or 
harder  the  iron,  the  greater  will  be  the  difference  between  the  curves 
C/?and.#  C. 

128.  Hysteresis.  —  The  difference  between  the  curves  is  caused  by 
the  iron  apparently  holding  on  to  its  preceding  magnetic  state,  and  the 
effect  is  called  Hysteresis,  as  suggested  by  Professor  J.  A.  Ewing,  during 
a  remarkable  series  of  investigations  and  discoveries  in  electromagnetism 


ELECTROMAGNETISM  1 3 1 

which  were  begun  in  1881.  One  of  the  causes  of  hysteresis  is  the  coer- 
cive force  in  the  iron,  and  one  of  its  effects  is  the  retention  of  residual 
magnetism  by  a  piece  of  magnetized  iron  after  the  magnetizing  force  has 
been  withdrawn. 

129.  Magnetic  Permeability.  —  From  what  1ms  preceded  we  may  see 
that  a  solenoid  in  which  a  current  flows  and  which  contains  a  soft  iron 
or  steel  core  is  a  stronger  magnet  than  a  similar  solenoid  containing  a 
hard  steel  core,  and  it  is  a  very  much  stronger  magnet  than  a  similar  sole- 


O  6000 
I 


I  8  10  12  14  13 

MAGNETIZING  FORCE 


FIG.  64.  —  Curve  of  Magnetization,  BC,  and  Curve  of  Demagnetization,  CD,  of  Soft  Iron. 

noid  containing  no  core.  Remembering  that,  according  to  our  idea  of 
lines  of  force,  the  strength  of  the  magnetism  of  the  solenoid  and  core 
depends  on  the  number  of  lines  of  force  which  pass  through  the 
solenoid  ;  then,  since  so  many  more  lines  of  force  pass  through  a  sole- 
noid when  a  hard  steel  bar  is  placed  in  it  than  pass  through  it  when  the 
space  within  the  solenoid  is  simply  occupied  by  air,  we  may  conclude 
that  lines  of  force  are  more  readily  set  up  in  steel  than  in  air ;  and  since 
a  soft  steel  core  causes  more  lines  of  force  to  pass  through  a  solenoid 


132 


ELECTRICITY  AND   MAGNETISM 


than  does  the  hard  steel  core,  we  may  also  conclude  that  lines  of  force 
are  still  more  readily  set  up  in  soft  steel. 

The  relative  ease  with  which  magnetic  lines  of  force  may  be  produced 
in  a  body  is  called  its  Permeability.  As  a  matter  of  convenience  it  is 
usual  to  say  that  the  permeability  of  air  is  unity  ( i ) .  As  compared  with 
this,  the  permeability  of  soft  iron  or  steel  is  very  great ;  it  may  be  several 
thousand  times,  and  in  some  cases  many  thousand  times,  as  great. 

The  permeability  of  all  materials,  except  a  few  highly  magnetic  ones, 
is  very  nearly  constant  at  the  value  of  unity,  or  the  same  as  that  of  air ; 


8000  12000 

LINES  OF  FORCE  PER  6Q.   CM. 


FIG.  65.  —  Permeability  Curves  of  Specimens  of  Wrought  Iron,  Soft  Steel  Castings,  and 

Cast  Iron. 

but  the  permeability  of  the  magnetic  materials  varies.  For  instance, 
the  permeability  of  iron  increases  rapidly  as  the  iron  is  being  mag- 
netized until  a  certain  magnetic  density  is  reached,  and  then  if  the 
magnetic  density  is  crowded  still  higher,  the  permeability  rapidly 
decreases.  Typical  "  permeability  curves  "  for  wrought  iron,  soft  steel 
castings,  and  cast  iron,  which  show  the  relation  between  magnetic  density 
and  permeability,  are  shown  in  Figure  65.  The  proper  division  between 


ELECTROMAGNETISM  133 

materials  that  are  called  paramagnetic  and  those  that  are  called  dia- 
magnetic1  depends  upon  whether  their  permeability  is  greater  than 
unity  or  is  slightly  less. 

We  may  divide  materials  into  good  conductors  of  magnetic  lines  of 
force,  or  good  magnetic  conductors,  and  poor  magnetic  conductors. 
There  are,  however,  no  materials  which  may  be  regarded  as  magnetic 
insulators,  in  the  way  that  we  regard  some  materials  as  being  practical 
insulators  of  electricity ;  and  all  materials,  and  a  vacuum  as  well,  are  to 
a  certain  degree  magnetic  conductors. 

The  permeability  of  a  material  may  be  called  its  Specific  Magnetic  Con- 
ductivity, that  is,  the  magnetic  conducting  power  of  a  block  of  the  material 
which  is  one  centimeter  long  and  has  an  area  of  one  square  centimeter. 
The  actual  magnetic  conductivity  of  any  piece  of  material  decreases  with 
the  length  and  increases  with  the  cross  section  of  the  piece.  This  may  be 
likened  to  electrical  conducting  power,  or  conductivity,  which  is  described 
in  Chapter  VII. 

PROBLEMS 

A.  How  many  units  of  magnetic  pressure  will  15  amperes  passing  through  a  coil 
of  30  turns  set  up?     Ans.  566  (approx.). 

B.  For  the  purpose  of  magnetizing  an  iron  bar  we  wish  1257  units  of  magnetic 
pressure,  we  have  a  coil  of  250  turns  and  10  ohms  resistance.     How  many  volts 
pressure  must  we  apply  to  the  coil?     Ans.  40  (approx.). 

C.  The  copper  wire  coil  of  an  electromagnet  has  1000  turns,  each  being  18  in. 
long.     The  wire  is  of  1050  circular  mils  cross  section.     If  the  coil  is  connected  to  a 
dynamo  which  furnishes  it  50  volts  pressure,  how  many  units  of  magnetic  pressure 
will  be  set  up?     Ans.  4190. 

D.  Four  Daniell  cells  are  connected  to  a  solenoid  of  1000  turns  and  4  ohms  resist- 
ance.    The  cells  each  set  up  a  pressure  of  i.i  volts  and  have  an  internal  resistance 
of  4  ohms.      How  much  magnetic  pressure  will  be  produced  when    the  cells  are 
arranged  with  2  sets  in  parallel  of  2  cells  in  series?    Ans.  346  units  (approx.). 

E.  If  in  Example  D  the  cells  had  all  been  in  series,  what  would  have  been  the 
magnetic  pressure?     Ans.  Nearly  276  units. 

F.  Two  coils  have  respectively  100  and  1000  turns  and  are  of  I  and  10  ohms 
resistance.      What  will  be  the  relative  magnetic   pressures  set  up   in  them    when 
the  coils  are  supplied  with  the  same  electric  pressure?     Ans.  They  will  be  equal. 

130.  Magnetic  Reluctance.  —  The  reciprocal  of  magnetic  conducting 
power  may  be  called  Magnetic  Resistance,  but  the  word  Reluctance  is 

1  Article  75. 


134  ELECTRICITY   AND   MAGNETISM 

usually  applied  to  it ;  and  any  path  through  which  lines  of  force  pass 
may  be  called  a  Magnetic  Circuit.  These  terms  may  be  recognized  as 
entirely  analogous  to  those  applied  to  the  case  of  electric  currents  an  1 
which  are  described  in  earlier  chapters. 

PROBLEMS 

A.  A  piece  of  iron  200  cm.  long  and  100  sq.  cm.  in  cross  section  has  a  per- 
meability of  1000  units  at  a  certain  magnetization.     What  is  the  reluctance  ?     (Aid : 
By  the  analogy  to  electric  resistance  we  can  write 

I  Length  in  centimeters 

Reluctance  =  —       — —=-. —  •  -FT  — i2- — = —  — = —  — » 

Permeability     Cross  section  in  square  centimeters 

or  using  symbols  to  represent  those  quantities, 

*--* 

V.A 

-  being  equivalent  to  specific  electrical  resistance.      Substituting  the  values  given  in 

M 

the  example,  the  result  is  easily  obtained.)     Ans.  .002  of  a  unit. 

B.  A  ring  of  soft  steel  has  a  permeability  of  500  at  a  given  magnetization,  and  a 
length  of  100  cm.     How  large  must  be  its  cross  section  if  its  reluctance  is  to  be 
.004  units?    Ans.  50  sq.  cm. 

C.  The  air  space  between  a  magnet  and  its  keeper  is  |  cm.  long  by  20  sq.  cm.  in 
cross  section.     What  is  the  reluctance  of  the  air  (/*=  i)  ?     Ans.  .025  unit. 

D.  A  ring  is  made  up  of  two  curved  bars,  one  of  cast  iron  having  a  permeability 
of  100,  and  the  other  of  wrought  iron  having  a  permeability  of  1000  at  a  desired 
magnetization.     Both  pieces  are  50  cm.  long  and  25  sq.  cm.  in  cross  section.     What 
is  the  reluctance  of  the  magnetic  circuit  formed  by  the  ring  at  the  given  magnet- 
ization?    (Aid:  Add  the  reluctances  of  the  two  pieces.)     Ans.  .022  unit. 

£.  A  magnetic  circuit  is  made  up  partly  of  a  curved  bar  of  iron  200  cm.  long 
by  50  sq.  cm.  in  cross  section,  and  having  a  permeability  of  1000  at  the  required 
magnetism,  and  partly  of  an  air  space  I  cm.  long  by  50  sq.  cm.  in  cross  section. 
What  is  the  reluctance  of  the  magnetic  circuit  at  the  required  magnetization?  Ans. 
.024  unit. 

131.  Magnetic  Pressure  and  the  Magnetism  set  up  by  It.  —  By  simi- 
larity with  electric  circuits  we  may  say  that  it  takes  some  force  to  set  up 
lines  of  force  in  a  magnetic  field  or  magnetic  circuit.  We  may  call 
this  Magnetomotive  Force  or  Magnetic  Pressure  (as  already  described 
in  Articles  89,  and  125),  terms  which  are  similar  to  electromotive 
force  and  electrical  pressure.  The  number  of  lines  of  force  in  any 
magnetic  circuit  is  equal  to  the  magnetic  pressure  divided  by  the  reluc- 


ELECTROM  AGNETISM  1  3  5 

tance  of  the  circuit  ;  and  it  can  be  shown  mathematically,  as  has  already 
been  set  forth,  that  the  magnetic  pressure  in  a  complete  magnetic  circuit 
is  equal  to  the  number  of  ampere  turns  multiplied  by  a  constant  which 
is  nearly  equal  to  \\.  This  may  be  set  down  in  the  form  of  a  formula, 
in  which  N  is  used  to  represent  the  number  of  magnetic  lines  of  force, 
M  the  magnetic  pressure,  P  the  magnetic  reluctance,  and  nc  the  number 
of  ampere  turns  : 


P  P 

In  order  that  the  strongest  possible  magnetism  shall  be  produced  in 
any  magnetic  circuit,  it  is  necessary  to  have  the  circuit  made  up  as  far 
as  possible  of  material  having  the  highest  permeability  —  that  is,  soft 
iron  —  and  to  arrange  as  many  ampere  turns  as  possible  to  set  up  the 
magnetism. 

The  apparent  similarities  of  electric  and  magnetic  circuits,  and  their 
really  fundamental  differences,  will  be  further  illustrated  in  later  articles. 

PROBLEMS 

A.  If  it  is  desired  to  have  a  piece  of  iron  magnetized  with  5000  lines  of  force  per 
square  centimeter,  what  cross  section  is  required  to  have  a  total  of  1,000,000  lines? 
Ans.  200  sq.  cm. 

B.  A  magnetic  circuit  composed  of  a  ring  of  iron  has  a  reluctance  of  .001  unit 
when  1,000,000  lines  of  force  pass  through  it.     How  much   magnetic  pressure  is 
required  to  create  the  1,000,000  lines?     (Aid:  From  the  relation  stated  above  we 
may  write 

Magnetic  Pressure 

Number  of  Lines  =  —  —  > 

Reluctance 

or  using  symbols  N  —  —  . 

Ans.  1000  units  of  pressure. 

C.  How  many   ampere   turns    would    be   required  in    Example   B  ?     Ans.    796 
(approx.). 

D.  A  ring  of  iron  250  cm.  long  and  50  sq.  cm.  in  cross  section  has  a  permeability 
of  500  when  10,000  lines  of  force  pass  through  it  per  square  centimeter.     How  much 
magnetic  pressure  will  be  required  to  set  up  500,000  lines?     (Aid:  First  find  the 
reluctance.)     Ans.  5000  units  of  pressure. 

E.  A  ring  of  iron  has  a  coil  of  200  turns  wound  upon  it.     The  ring  is  100  cm. 
long  by  20  sq.  cm.  in  cross  section,  and  has  a  permeability  of  500  when  50,000  lines 
of  force  pass  through  it.     How  much  current  will  be  required  to  set  up  this  mag- 
netization?    Ans.  1.98  amperes  (approx.). 


136  ELECTRICITY   AND   MAGNETISM 

F.  A  ring  of  iron,  200  cm.  long,  is  to  carry  a  total  of  500,000  lines  of  force  with  a 
magnetic    density  of  5000  lines  per  square  centimeter.      The  permeability  at  that 
magnetization  is  1000.     How  many  turns  will  be  required  in  the  magnetizing  coil 
carrying  2  amperes?     Ans.  399  (approx.). 

G.  A  ring  is  made  up  of  two  curved  bars,  one  of  cast  iron  having  a  permeability 
of  100,  and  the  second  of  wrought  iron  having  a  permeability  of  1000  at  an  induction 
of  5000  lines  per  square  centimeter.     Both  pieces  are  50  cm.  long  and  25  sq.  cm. 
in  cross  section.     How  many  ampere  turns  will  be  required  to  set  up  5000  lines  of 
force  per  square  centimeter?     (Aid:    Find  reluctance  and  total  number  of  lines.) 
Ans.  2188  (approx.). 

H.  A  magnetic  circuit  is  made  up  partly  of  a  curved  bar  of  iron  200  cm.  long  by 
50  sq.  cm.  in  cross  section,  and  having  a  permeability  of  1000  at  a  magnetic  induction 
of  10,000  lines  of  force  per  square  centimeter  cross  section  ;  the  remaining  part  is  an 
air  space  I  cm.  long  by  50  sq.  cm.  cross  section.  What  magnetic  pressure  is  required 
to  set  up  a  total  of  500,000  lines  of  force?  Ans.  12,000  units. 

/.  In  Example  //  how  many  units  of  pressure  were  required  to  force  the  lines  of 
force  through  the  iron?  Ans.  2000. 

J.  If  the  cross  section  of  the  magnetic  circuit  in  Example  H  was  doubled,  how 
many  lines  of  force  would  12,000  units  of  magnetic  pressure  set  up?  Ans.  1,000,000. 

K.  If  a  coil  of  2000  turns  and  10  ohms  resistance  be  used  in  Example  //  for 
obtaining  the  magnetic  pressure,  how  many  volts  must  be  applied  to  the  coil  ? 
Ans.  47.7  (approx.). 

QUESTIONS 

39.  What  happens  to  a  bar  of  steel  placed  within  a  solenoid? 

40.  What  is  unit  magnetic  pressure? 

41.  How  many  ampere  turns  are  required  to  set  up  unit  magnetic  pressure? 

42.  How  much  magnetic  pressure  will  be  set  up  by  one-hundredth  of  an  ampere 
in  a  coil  of  one  hundred  turns? 

43.  How  much  magnetic  pressure  will  be  set  up  by  I  ampere  in  a  coil  of  ten  turns  ? 

44.  What  happens  to  a  bar  of  soft  iron  when  it  is  placed  in  a  solenoid? 

45.  What  happens  in  Questions  39  and  44  if  the  current  is  shut  off? 

46.  What  is  residual  magnetism? 

47.  What  is  an  electromagnet? 

48.  Which    will   become    the   most   highly   magnetized   by   the   same    magnetic 
pressure,  hard  steel  or  soft  iron? 

49.  Describe  the  form  and  construction   of  any  electromagnet  with  which  you 
may  be  familiar. 

50.  What  characteristic  of  soft  iron  did  William  Sturgeon  discover?     When? 

51.  Outline  a  history  of  the  electromagnet. 

52.  What  is  a  curve  of  magnetization? 

53.  If  the  current  in  an  electromagnet  be  increased   I   ampere  at  a  time  up  to 
50  amperes,  will  the  magnetism  increase  proportionately  to  the  current? 


ELECTROM  AGNETISM  1 3  7 

54.  If  in  Question  53,  25  amperes  saturated  the  iron,  what  would  be  the  effect 
of  the  last  25  amperes? 

55.  If  in  Question  53  the  current  be  decreased  again,  will  the  curve  of  magnetism 
be  the  same  as  for  the  rising  current?     Will  it  be  higher  or  lower? 

56.  What  is  hysteresis? 

57.  What  causes  hysteresis?     How? 

58.  Which  will  a  solenoid  set  up  lines  of  force  in  most  readily,  air,  soft  iron,  or 
hard  steel?     Which  next  best? 

59.  What  is  permeability  ? 

60.  Compare  magnetic  permeability  to  specific  conductivity  of  electric  conductors. 

61.  What  is  the  permeability  of  air,  wood,  brick,  etc.? 

62.  What  are  paramagnetic  and  diamagnetic  bodies? 

63.  Are  there  any  magnetic  insulators? 

64.  What  effect  have  the  length  and  cross  section  on  the  magnetic  conductivity  of 
a  piece  of  iron? 

65.  W7hat  is  magnetic  reluctance? 

66.  How  does  magnetic  reluctance  compare  with  electrical  resistance? 

67.  What  is  a  magnetic  circuit? 

68.  If  one  magnetic  circuit  is  twice  as  long  and  has  tuice  the  cross  section  of 
another  made  of  the  same  material,  will  they  be  of  equal  reluctance? 

69.  If  a  certain  ring  of  steel  has  one-tenth  the  permeability  of  an  equal  ring  of 
iron,  how  much  greater  will  be  the  reluctance  of  the  steel  than  that  of  the  iron? 

70.  Compare  magnetic  pressure  with  electric  pressure. 

71.  Construct  a  law  for  the  magnetic  circuit  similar  to  Ohm's  Law. 

72.  A  certain  ring  of  iron  has  a  reluctance  of  2.5  units.    A  coil  of  wire  wrapped 
around  it  sets  up  2500  units  of  magnetic  pressure.     How  many  lines  of  force  will  be 
.created  in  the  iron? 


CHAPTER    X 


ELECTROMAGNETIC    INDUCTION 

132.  Pressure  induced  by  moving  a  Conductor  across  a  Magnetic 
Field.  —  The  experiments  of  Oersted,  Davy,  Ampere,  Arago,  Sturgeon, 
and  others  showed  the  intimate  relation  existing  between  electricity  and 
magnetism,1  and  also  showed  that  the  flow  of  an  electric  current  always 
produces  magnetism.  It  remained  for  the  brilliant  experimental  studies 
of  Michael  Faraday,  of  the  Royal  Institution,  London,  and  of  Professor 
Joseph  Henry,  of  Princeton  College,  New  Jer- 
sey, to  make  the  most  important  additions  to 
our  knowledge  of  the  mutual  action  between 
electric  currents  and  magnetism.  Within  two 
years  after  the  publication  by  Oersted  of  the 
discovery  that  a  magnetic  needle  may  be  de- 
flected by  bringing  near  it  a  wire  carrying  an 
electric  current,  Faraday  had  succeeded  in  pro- 
ducing a  continuous  motion  by  means  of  the 
effect  of  an  electric  current  upon  a  permanent 
magnet ;  and  it  was  soon  after  learned  that  a 
wire  hung  over  the  pole  of  a  magnet  and  with 
its  ends  in  mercury  troughs,  as  shown  in  Figure 
66,  would  continuously  revolve  around  the  pole 
on  account  of  the  mutual  attraction  between 

the  lines  of  force  belonging  to  the  magnet  and  to  the  current  in  the 
wire.2  In  the  motion  thus  produced  lies  the  principle  of  the  operation 
of  the  electric  motors  and  many  of  the  electrical  instruments  which 
prove  so  useful  at  the  present  day. 

The  best  method  of  generating  an  electric  current  at  this  time  was  by 
means  of  an  electric  battery,  and  the  usefulness  of  the  electric  motor 

1  Chapter  IX.  2  Article  123. 

138 


FIG.  66.  —  Wire  W,  ar- 
ranged to  rotate  around 
Magnet  Pole  A^S,  through 
the  Mutual  Action  of 
Current  and  Magnet. 


ELECTROMAGNETIC   INDUCTION 


139 


could  be  but  small  as  long  as  it  depended  for  its  power  upon  the  con- 
sumption of  zinc  in  a  battery.1  To  the  vigorous  minds  of  P'araday  and 
Henry,  the  production  of  motion  when  an  electric  current  was  brought 
under  the  influence  of  a  magnet,  seemed  to  suggest  a  reverse  action 
through  which  an  electric  current  might  be  produced  by  the  motion  of 
a  wire  in  a  magnetic  field.  This  thought  led,  shortly  after  1830,  to  the 
magnificent  discovery  by  Faraday  that  a  tendency  for  electric  currents  to 
flow  is  produced  in  a  conductor  when  it  is  moved  in  a  magnetic  field  so 
as  to  cut  through  the  lines  of  force  of  the  field.  That  is,  an  electric 
pressure  is  set  up  in  the  conductor  when  it  cuts  the.  lines  of  force.  The 
two  great  experimenters  also  independently  discovered  the  fact  that  any 
change  in  the  magnetic  field  around  a  wire  tends  to  set  u'p  an  electric  cur- 
rent in  the  wire,  exactly  as  any  change  in  an  electric  current  which 
flows  in  a  wire  causes  a 
corresponding  change  in 
the  magnetic  field  about  it. 

In  this  great  discovery 
lies  the  principle  of  the 
operation  of  Dynamo  Elec- 
tric Generators  or  Dyna- 
mos, as  they  are  usually 
called.  Faraday  himself 
in  1831  made  what  may 
be  called  the  first  model 
of  a  dynamo.  This  consisted  of  a  disk  of  copper  rotated  between  the 
poles  of  a  strong  magnet  (Fig.  67).  From  this  disk  a  current  was 
collected  by  copper  brushes  which  rubbed  on  the  edge  of  the  disk  and 
on  its  shaft. 

133.  Early  Magneto  Machines. — Faraday's  discovery  was  quickly 
turned  into  commercial  service,  as  will  be  described  in  a  later  chapter, 
and  many  smaHmachines  were  made  for  generating  electric  currents  by 
rotating  coils  of  wire  between  the  poles  of  permanent  magnets.  These 
machines  with  permanent  magnets  are  ordinarily  spoken  of  as  Magneto- 
electric  Generators  or  Magnetos  to  distinguish  them  from  the  ordinary 
dynamo  electric  generators  or  dynamos  which  have  electromagnets. 

1  Refer  to  Article  54. 


FiG.  67.  —  Faraday's  Disk  Dynamo. 


140  ELECTRICITY   AND   MAGNETISM 

The  magnetos  which  are  used  for  ringing  telephone  call  bells  belong 
to  the  same  class  as  the  early  machines. 

134.  Magnitude  of  Pressure  set  up  in  a  Moving  Wire.  —  When  a 
wire  is  moved  in  a  magnetic  field  so  that  it  cuts  lines  of  force,  the 
action  which  occurs  causes  a  difference  of  electric  pressure  between  the 
two  ends  of  the  wire.  The  magnitude  and  direction  of  the  pressure 
which  is  thus  Induced  depends  upon  certain  fixed  relations. 

The  magnitude  of  the  pressure  depends  upon  the  rate  at  which  the 
wire  cuts  lines  of  force,  that  is,  upon  the  total  number  of  lines  of  force 
cut  by  the  wire  in  a  second  of  time.  When  the  wire  cuts  one  hundred 

million     (100,000,000) 

•f- — ' 7 lines  of  force  in  every 

second  during  its  mo- 
tion, an  electric  press- 
ure of  one  volt  is  set 
up,  and  if  the  wire  (like 
/  B  C  in  Fig.  68)  is  laid 

FIG.  68.  —  Illustration  of  Rails  with  Slider  to  cut  Lines       across    conducting    rails 
of   Force.  "     ° 

which  are  electrically 

connected  through  a  galvanometer  (shown  at  G  in  the  figure)  the  gal- 
vanometer will  indicate,  while  the  wire  moves,  the  flow  of  a  current 
which  has  a  strength  equal  to  the  induced  pressure  divided  by  the 
resistance  of  the  electric  circuit  made  up  of  the  galvanometer,  rails,  and 
moving  wire. 

If  the  wire  cuts  through  the  lines  of  force  at  the  rate  of  two  hundred 
millions  (200,000,000)  to  the  second,  the  induced  pressure  is  equal  to 
two  volts,  and  if  the  wire  cuts  only  seventy-five  million  (75,000,000) 
lines  each  second,  a  pressure  of  only  three- fourths  (f)  volts  is  set  up, 
and  so  on,  which  is  according  to  the  rule  given  above. 

The  number  of  lines  of  force  which  are  cut  in  a  second  by  a  wire 
moving  in  a  magnetic  field  depends  upon  four  items. 

1.  Upon  the  strength  of  the  field,  or  the  number  of  lines  of  force 
which  it  contains  in  each  square  centimeter. 

2.  Upon  the  length  of  the  wire  which  is  in  the  field. 

3.  Upon  the  speed  with  which  the  wire  moves. 

4.  Upon  the  angle  with  which  the  wire  moves  across  the  lines  of 


ELECTROMAGNETIC   INDUCTION  141 

force.  If  the  wire  moves  diagonally  across  the  lines  of  force  it  does 
not  cut  through  as  many  lines  in  a  given  time  as  when  it  moves 
equally  fast  at  right  angles  to  the  lines. 

PROBLEMS 

A.  A  wire  cuts  at  a  constant  rate  of  speed  through  a  field  of  1,000,000  lines  of 
force,  2400  times  per  minute.      How  many  volts  are  set  up  in  the  wire?     Ans.  .4 
of  a  volt. 

B.  A  wire  100  cm.  long  passes  through  a  magnetic  field  having  an  induction  of 
10,000  lines  of  force  per  square  centimeter.     If  it  travels  at  the  rate  of  1000  cm.  per 
second  at  right  angles  to  the  lines,  what  pressure  will  be  induced?      (Aid:    Find 
number  of  lines  cut  per  second.)     Ans.  10  volts. 

C.  Twenty  conductors,  connected  so  that  their  induced  pressures  are  in  series,  cut 
through  5,000,000  lines  of   force  at  the  constant  rate  of   3,000  times  per  minute. 
What  is  the  average  pressure  set  up  by  the  set  of  conductors?     Ans.  50  volts. 

D.  One  side  of  a  large  coil  of  wire   having  100  turns,  cuts  through   a   field  of 
10,000,000  lines  of  force  at  an  average  rate  of  2400  times  per  minute.      What  is 
the  average  pressure  developed?    (Aid:  The  turns  are  in  series.)     Ans.  400  volts. 

E.  A  coil  of  one  turn  includes  5,000,000  lines  of  force  within  its  area  when  the 
plane  of  its  face  is  at  right  angles  to  the  lines.     If  it  is  turned  over  at  the  rate  of 
1 200  revolutions  per  minute,  what  will  be  the  average  of  the  pressure  induced  during 
each  half  revolution  ?     (Aid :    each    side  of  the   coil  cuts  all  the  lines  twice  in  a 
revolution.)     Ans.  4  volts. 

F.  If  the  coil  in  Example  E  had  25  turns,  what  would  be  the  average  pressure 
induced?    Ans.  100  volts. 

G.  If  a  slider,  when  cutting  at  right  angles  across  a  magnetic  field  at  a  speed  of 
25  ft.  per  minute,  induces  2  volts,  how  fast  must  it  go  to  induce  the  same  pressure 
when  its  direction  is  so  inclined  to  the  lines  that,  at  a  speed  of  25  ft.  per  minute,  it 
only  induces  |  of  a  volt?     Ans.   100  ft.  per  minute. 

//.  A  slider  100  cm.  long  cuts  at  right  angles  across  a  field  of  5000  lines  of 
force  per  square  centimeter.  If  the  resistance  of  the  slider  is  .5  of  an  ohm  and  its 
ends  are  joined  by  a  wire  of  negligible  resistance,  how  much  current  will  flow  when 
it  is  moving  at  the  rate  of  300  cm.  per  second?  Ans.  3  amperes. 

7.  How  much  power  (in  watts)  is  required  in  Example  H  to  push  the  slider, 
supposing  there  is  no  friction?  Ans.  4.5  watts. 

J.  One  side  of  a  coil  of  fifty  turns  cuts  through  5,000,000  lines  of  force  at  a  speed, 
during  the  moment  considered,  of  2400  times  per  minute.  The  resistance  of  the  coil 
is  .  2  of  an  ohm,  and  the  resistance  of  the  external  circuit  to  which  it  is  attached  is 
.8  of  an  ohm.  How  much  current  flows?  Ans.  100  amperes. 

K.  How  many  horse  power  are  required  to  drive  the  coil  in  Example  y?  Ans. 
13.4  (approx.). 


142  ELECTRICITY   AND   MAGNETISM 

Z.  How  much  pressure  is  used  in  the  coil  and  how  much  in  the  external  circuit  of 
Example/?  Am,  20  volts.  80  volts. 

M.    What  is  the  pressure  at  the  terminals  of  the  coil  in  Example/  ?  Ans.  80  volts. 

N.  What  power  is  used  in  the  coil  and  what  in  the  external  circuit  of  Example/  ? 
Ans.  2000  watts.  8000  watts. 

O.  How  many  Daniell  cells  of  2  ohms  internal  resistance  and  I  volt  pressure  must 
be  used  to  equal  the  power  of  the  arrangement  in  Example/,  supposing  that  the 
internal  resistance  of  the  battery  equals  the  resistance  of  the  coil  ?  How  should  they 
be  arranged  ?  Ans.  100,000  cells.  1000  rows  in  parallel,  each  of  100  cells  in  series. 

135.  Direction  of  Induced  Pressure  in  the  Moving  Wire.  —  The  di- 
rection of  the  induced  electric  pressure  depends  upon  the  direction  of  the 
lines  of  force  in  the  magnetic  field  and  the  direction  in  which  the  wire 

cuts  through  them.      In  Figure  69, 
if  the  vertical  arrows  show  the  di- 
rection of  the  lines  of  force   and 
the  horizontal  arrow  which  lies  be- 

*    ,  tween  the  rails  shows  the  direction 

FIG.  69.  —  Slider  and  Rails. 

in  which  the  wire  AB  moves,  then 

the  end  B  of  the  moving  wire  is  positive  and  the  other  end  negative  in 
pressure.  That  is,  a  current  will  flow  around  the  circuit,  composed  of 
the  wire  and  the  rails,  from  B  through  C  and  D  to  A  and  from  A 
through  the  wire  to  B. 

The  current  flows  in  the  external  circuit,  BCD  A,  from  the  positive 
or  high  pressure  end  to  the  negative  or  low  pressure  end  of  the  wire,  and 
within  the  moving  wire  the  current  flows  from  the  low  pressure  end  to 
the  high  pressure  end. 

The  motion  of  the  wire  across^  the  lines  of  force  causes  it  to  act  like  a 
pump,  which  lifts  the  electric  current  from  its  low  pressure  or  suction 
end  to  its  high  pressure  or  discharge  end.  In  this  respect  the  moving 
wire  acts  exactly  like  a  friction  machine  or  a  primary  battery,  such  as 
are  described  in  Chapters  III  and  IV. 

If  the  direction  of  the  wire's  motion  is  reversed,  the  direction  of  the 
current  is  also  reversed.  Reversing  the  direction  of  the  lines  of  force 
also  reverses  the  current. 

There  are  various  ways  of  remembering  the  relation  between  the 
direction  of  the  electric  current,  the  direction  of  the  wire's  motion,  and 


ELECTROMAGNETIC   INDUCTION 


143 


the  direction  of  the  lines  of  force.     One  of  them  is  to  hold  up  the 

right  hand,  with  the  thumb  pointing  straight  up,  the  first  finger  pointing 

straight  out,  and  the  middle  finger 

turned   off  to   the    left    (Fig.   70). 

Now,  if  the  hand  is  turned  in  such 

a  direction  that  the  Thumb  points 

in  the   direction  of  Motion  of  the 

wire,  and  the  First  Finger  points 

in    the    direction    of   the    lines    of 

Force,  then  the  middle  or   Central 

Finger  will  point  in  the  direction  of 

the  Current  which  is  set  up  in  the 

wire  by  the  induced  pressure. 

Another  way  of  remembering  this 
relation  is  by  a  modification  of  Am- 
pere's rule.1  If  a  man  lies  in  the 
moving  conductor  so  that  he  looks 


FIG.  70.  —  Illustration  of  "  Right-hand 
Rule  "  for  showing  Direction  of  In- 
duced Current. 


down  along  the  lines  of  force  (his 

face  is  then  toward  the  south  pole} ,  and  the  motion  is  toward  his  right 
hand,  he  will  be  floating  head  first  down  the  current  which  is  set  up  in 
the  wire. 

136.  Experimental  Illustrations.  —  It  has  already  been  explained2 
that  the  earth  is  a  great  magnet,  and  that  its  lines  of  force,  therefore, 
reach  out  through  all  the  space  within  which  we  live.  The  induction 
of  electric  pressure  by  a  wire  cutting  lines  of  force  may,  therefore,  be 
illustrated  by  swinging  a  long  wire  in  the  earth's  magnetic  field.  If  a 
wire  is  suspended  across  a  room,  and  its  ends  are  attached  to  a  sensitive 
galvanometer,  the  needle  of  the  galvanometer  will  be  deflected  from 
side  to  side  when  the  wire  is  set  to  swinging.  When  the  wire  moves  in 
one  direction,  the  needle  will  move  to  one  side  of  its  zero  point ;  and 
when  the  wire  moves  in  the  other  direction,  the  needle  will  move  to  the 
other  side  of  the  zero.  This  shows  that  the  direction  of  the  pressure 
induced  by  the  cutting  of  the  earth's  lines  of  force  depends  upon  the 
direction  in  which  the  wire  moves  across  the  lines. 

If  the  wire  is  caused  by  some  means  to  swing  more  slowly,  the  deflec- 
1  Article  122.  2  Article  90. 


144  ELECTRICITY   AND    MAGNETISM 

tions  of  the  galvanometer  needle  will  be  smaller,  showing  that  the  mag- 
nitude of  the  induced  pressure  depends  upon  the  velocity  of  motion  of 
the  wire. 

If  half  the  wire  is  now  replaced  by  a  piece  of  string,  and  the  ends  of 
the  remaining  half  are  connected  to  the  galvanometer  without  practi- 
cally altering  the  resistance  of  the  circuit,  and  the  wire  is  set  swinging 
at  about  the  same  speed  as  before,  the  galvanometer  deflections  are 
reduced  to  about  one-half  their  former  value,  showing  that  the  induced 
pressure  depends  upon  the  length  of  the  wire. 

These  experiments  can  only  be  successfully  carried  out  in  some  such 
favorably  equipped  place  as  a  school  laboratory,  but  their  description 
serves  to  illustrate  the  effect  of  moving  a  conductor  across  magnetic 
lines  of  force.  An  experiment  illustrating  the  same  thing  may  be  made 
by  means  of  a  permanent  magnet,  a  coil  of  wire,  and  any  galvanometer 
with  a  light  needle  which  is  obtainable.  If  the  coil,  made  up  of  a  few 
turns  of  wire,  is  slipped  along  one  end  of  the  magnet  at  a  fixed  speed, 
the  galvanometer  needle  will  show  a  certain  deflection.  Now,  if  more 
turns  are  added  to  the  coil,  which  is  then  moved  exactly  as  before,  the 
galvanometer  deflection  will  be  proportionally  greater,  showing  that  a 
greater  electric  pressure  has  been  induced. 

In  the^  case  of  the  coil  we  have  the  following  condition  :  each  turn 
cuts  the  lines  of  force  at  a  certain  rate  a.3  the  coil  is  slipped  along  the 
magnet,  and  a  corresponding  electric  pressure  is  set  up  in  it.  Since 
the  turns  of  the  coil  are  all  connected  in  series,  and  the  electric  press- 
ures- set  up  in  them  are  all  in  the  same  direction,  the  electric  pressure 
induced  in  the  whole  coil  is  equal  to  the  sum  of  the  pressures  developed 
in  all  of  its  turns.  This  is  exactly  similar  to  the  case  of  an  electric 
battery  with  its  cells  connected  in  series,  where  the  battery  pressure  is 
equal  to  the  sum  of  the  pressures  of  all  the  cells.  Adding  additional 
cells  to  the  battery  increases  the  battery  pressure,  and  adding  additional 
turns  to  the  moving  coil  increases  the  total  pressure  induced  in  it. 

If  the  connections  of  some  of  the  cells  in  the  battery  are  reversed, 
the  pressure  at  the  battery  terminals  is  reduced,  and  becomes  equal  to 
the  difference  of  the  pressures  which  are  developed  by  the  cells  con- 
nected in  one  way  and  those  which  are  connected  in  the  reverse  way. 
In  the  same  way,  if  part  of  the  turns  of  the  moving  coil  be  wound  in 


ELECTROMAGNETIC   INDUCTION 


145 


fi 


r 

1 

k 

/I             ///    55 

1    Vv\  iii 

i 

i 

5_s§2 

s 

j 

rz 

i\\  //* 

one  direction,  and  part  in  the  other  direction,  the  pressures  developed 
in  the  two  parts  are  opposite,  and  the  effective  pressure  developed  by 
the  coil  is  equal  to  the  difference  of  the  pressures  which  are  developed 
in  the  parts.  If  half  the  turns  are  right-handed  and  half  left-handed, 
no  current  will  flow  in  the  coil  when  it  moves  in  the  magnetic  field, 
because  the  pressure  developed  in  one  half  of  the  turns  tends  to  cause 

the  current  to  flow  one  way,    <         K~^    

and  the  equal  pressure  de- 
veloped in  the  other  half  of 
the  turns  tends  to  cause  the 
current  to  flow  in  the  oppo- 
site direction.  The  two  ten- 
dencies neutralize  each  other, 
and  no  current  flows. 

For  the  same  reason,  if  a 
coil  of  wire  be  moved  straight    -^ 

across   the   lines  of  force   of   a    FIG.  71.  —  Coil  moved  parallel  to  itself  in  a  Uni- 
. ,.  ,-    •.  j     /T-,.  form  Magnetic   Field.     The  arrows  show  the 

uniform    field    (Fig.    71),    no          direction/of  Induced  electrical  pressures. 
current  will  flow  in  the  coil, 

since  the  pressures  developed  in  the  two  halves  of  each  turn  are  in 
opposition,  as  shown  by  the  arrows,  and  are  of  equal  value.  The  truth 
of  this  may  be  easily  proved  by  applying  one  of  the  rules  given  in  the 

preceding  article. 

If  the  coil  is  mounted 
on  an  axis  or  shaft,  so 
that  it  may  be  revolved 
in  the  field  (Fig.  72),  a 
different  condition  ex- 
ists.      Now,    the    two 
halves  of  the  coil  cut 
the   lines   of    force   in 
such    a   way    that    the 
pressures    are    in    the 
same  direction  as  shown  by  the  arrows,  and  a  current,  therefore,  flows 
in   the   coil.      Figure    73  shows  the   coil  after  it  has   turned   through 
a  half  revolution  from  its  first  position.     From  this  figure  it  is  seen  that 
L 


FIG.  72.  —  Coil  revolved  in  Magnetic  Field. 


146  ELECTRICITY  AND    MAGNETISM 

the  two  sides  of  the  coil  are  now  both  cutting  the  lines  of  force  in  a 
direction  which  is  opposite  to  that  in  which  they  cut  the  lines  before. 
The  direction  of  the  current  in  the  coil  is  therefore  reversed.  As 

the  coil  continues  re- 
volving, the  current  in 
it  is  reversed  in  every 
half  revolution.  Such 
a  current,  which  flows 
first  in  one  direction 
and  then  in  another, 
is  called  an  Alternat- 

FlG.  73. —  Coil  revolved  in  Magnetic  Field. 

ing  Current. 

137.  Methods  by  which  Pressures  may  be  Induced.  —  It  has  been  ex- 
perimentally proved  that  any  change  in  the  magnetic  field  around  an 
electric  conductor,  which  causes  the  lines  of  force  to  cut  the  conductor, 
tends  to  cause  an  electric  current  to  flow  in  the  conductor.1  We  are 
now  sufficiently  acquainted  with  the  mutual  effects  of  electric  currents 
and  magnetism,  so  that  it  is  not  surprising  to  learn  that  there  are  various 
conditions  under  which  the  effects  of  magnetism  may  result  in  an  elec- 
tric current.  One  of  these  conditions  is  seen  where  the  motion  of  a 
conductor  across  magnetic  lines  of  force  causes  a  current  to  flow  in  the 
electric  circuit,  of  which  the  conductor  is  a  part.  It  is  not  necessary 
that  the  conductor  move,  but  the  magnetic  field  may  move,  so  that  its 
lines  of  force  cut  across  stationary  conductors.  In  fact,  an  electric 
pressure  is  set  up  in  a  conductor  when  it  cuts  lines  of  force,  whe 'flier  the 
cutting  be  caused  by  the  motion  of  the  conductor,  or  by  the  motion  of  the 
lines  of  force. 

The  magnetic  lines  of  force  which  are  cut  by  a  conductor  and  so 
cause  an  electric  pressure  in  the  conductor  may  not  come  from  a 
magnet,  but  may  belong  to  an  electric  current  in  a  neighboring  wire. 
When  a  conductor  is  moved  toward  or  away  from  a  wire  carrying  a 
current,  the  lines  of  force  belonging  to  the  current  are  cut  by  the 
moving  conductor  and  an  electric  pressure  is  induced  in  it.  If 
the  wire  carrying  the  current  is  moved  toward  or  away  from  the  other 
conductor,  the  lines  of  force  belonging  to  the  current  cut  the  con- 

1  Article  132. 


ELECTROMAGNETIC   INDUCTION 


FIG.  74.  —  Primary  and  Secondary  Coils  for  induc- 
ing a  Current  by  moving  Another  Current. 


ductor  which  is  now  stationary,  and  an  electric  pressure  is  set  up  as 
before. 

The  wire  carrying  the  current  may  be  in  the  form  of  a  coil,  like  P  in 
Figure  74.  An  electric  pressure  may  be  set  up  in  the  conductors  of 
another  coil,  S,  by  simply 
thrusting  the  first  coil  which 
carries  a  current  into  the 
second.  After  the  Primary 
coil  P  is  pushed  into  the 
Secondary  coil  6*  and  its 
movement  is  stopped,  the 
electric  current  in  the  sec- 
ondary also  stops  because 
the  conductors  no  longer 
cut  lines  of  force  and  the 
electrical  pressure  is  no 
longer  produced.  Now  if 

the  primary  coil  is  drawn  out  from  the  secondary  coil,  an  electrical 
pressure  is  again  set  up  in  the  latter.  This  presstire  and  its  resulting 
current  is  opposite  to  that  set  up  when  the  primary  coil  was  pushed  into 
the  secondary,  because  the  lines  of  force  are  cut  in  the  opposite  direction 
by  the  secondary  coil. 

The  same  effects  may  be  produced  by  moving  a  secondary  coil  in  and 
out  of  a  larger  primary  coil,  or  by  moving  one  coil  around  near  the 
other.  The  battery  cell  C,  shown  in  Figure  74,  furnishes  current  to 
the  coil. 

QUESTIONS 

1.  Give  a  brief  history  of  the  development  of  electromagnetic  induction. 

2.  What  seven  men  were  most  closely  allied  with  the  historical  development  of 
electromagnetic  induction  ?     What  did  they  do  ? 

3.  How  can  a  magnet  be  given  a  continuous  motion  by  means  of  an   electric 
current  ? 

4.  How  can  a  wire  bearing  a  current  be  arranged  so  that  it  will  revolve  contin- 
uously upon  a  magnet  pole  ? 

5.  Why  were  not  motors,  made  as  in  Question  4,  useful  in  Faraday's  day  ? 

6.  What  facts  led  Faraday  and  Henry  to  believe  that  an  electric  current  could  be 
induced  ? 


148  ELECTRICITY   AND   MAGNETISM 

7.  State  Faraday's  discovery  with  reference  to  a  wire  moving  in  a  field.     When 
was  it  made  ? 

8.  State  the  effect  of  any  change  of  magnetic  field  about  a  wire.      Who  dis- 
covered this  ? 

9.  Describe  Faraday's  first  dynamo.     When  did  he  make  it  ? 

10.  What  are  magnetos  ? 

11.  Which  is  induced,  the  current  or  pressure  ? 

12.  What  does  an  electromagnetically  induced  pressure  depend  upon  primarily  ? 

13.  How  many  lines  of  force  must   be   cut   per  second  to  create  a  pressure  of 
I  volt  ? 

14.  Suppose  the  rate  of  cutting  lines  was  one  hundred  millions  in  2  seconds, 
what  would  be  the  induced  pressure  ? 

15.  Suppose   the    rate    of   cutting   lines  was  one  hundred  millions  in  |   second, 
what  would  be  the  induced  pressure  ? 

1 6.  Does  Ohm's  Law  apply  to  circuits  having  a  constant  induced  pressure  ? 

17.  Upon  what  three  things  does  the  number  of  lines  cut  by  a  conductor  moving 
in  a  magnetic  field  depend  ? 

18.  With  the  slider  and  rail  arrangement  described  in  Article  134,  how  much  more 
pressure  will  be  induced  if  the  original  length  of  the  slider  be  doubled  ?    If  the  lines 
per  square  centimeter  be  doubled  ? 

19.  Upon  what  does  the  direction  of  an  induced  pressure  depend  ? 

20.  Will  the  direction  of  an  induced  pressure  be  reversed  by  reversing  the  direction 
of  movement  ?     By  reversing  the  field  ? 

21.  Will  the  induced   pressure  be  reversed  by  reversing  the  clire:tions  of  both 
the  movement  and  the  lines  of  force  ? 

22.  Does  the  current  tend  to  flow  away  from  or  toward  the  point  of  high  pressure 
in  a  conductor  within  which  a  current  is  being  induced  ?     How  is  it  in  the  remainder 
of  the  circuit  ? 

23.  Is  pressure  used  up  within  a  conductor,  having  an  induced  pressure,  by  the 
induced  current  flowing  through  its  resistance  ? 

24.  How  can  you  show  experimentally  that  part  of  the  total  pressure  induced  in 
a  conductor  is  used  in  driving  a  current  through  its  own  resistance  ? 

25.  Compare  the  slider  and  rail  arrangement  with  a  pump  and  also  with  a  battery. 

26.  Give   the  "hand"   rule   for    determining   the    relative    direction   of  motion, 
current,  and  lines  of  force. 

27.  Give  the  "  swimming  "  rule  for  relative  direction  of  motion,  current,  and  lines 
of  force. 

28.  Illustrate  the  fundamental  laws  of  electromagnetic  induction  by  a  long  swing- 
ing wire.     Devise  an  illustration  for  yourself. 

29.  Illustrate,  as  in  Question  28,  by  a  magnet  and  coil  of  wire. 

30.  Why  does  increasing  the  number  of  turns  in  a  coil  increase  the  amount  of 
pressure  which  is  induced  in  it  when  a  magnetic  field  is  caused  to  thread  the  coil  ? 


ELECTROMAGNETIC   INDUCTION  149 

31.  Why  will  a  coil  of  wire  moved  across  a  uniform  magnetic  field  not  have  a 
pressure  induced  within  it  ? 

32.  What  is  an  alternating  current  ? 

33.  Why  will  a  coil  revolved  in  a  magnetic  field  set  up  an  alternating  current  ? 

34.  Is   it    necessary  that  a   conductor  shall  move    to  have  a    pressure  induced 
within  it? 

35.  May  the  magnetism  set  up  by  a  current  in  a  wire  be  made  to  induce  a  current 
in  another  circuit  ? 

36.  What  happens  when  a  coil  carrying  a  current  is  pushed  into  another  coil  ? 

37.  What  happens  when  the  inner  coil  of  Question  36  is  withdrawn  ? 

138.  Mutual  Induction.  —  The  effects  described  in  Article  137  may  be 
produced  by  fixing  the  coil  P,  inside  of  the  coil  S,  and  then  varying  the 
current  which  flows  through  the  coil  P.     When  the  current  increases  in 
the  primary  coil,  the  lines  of  force  belonging  to  the  magnetic  field  of 
the  current  cut  the  conductors  of  the  secondary  coil  as  they  are  pro- 
duced, and  thus  set  up  an  electric  pressure  in  the  secondary  coil  during 
the  time  the  magnetic  field  is  increasing.     If  the  Primary  Current  is  re- 
duced or  shut  off  entirely,  an  electric  pressure  is  set  up  in  the  secondary 
coils  in  the  opposite  direction  during  the  time  that  the  magnetic  field  is 
decreasing. 

Electric  currents  which  are  set  up  in  circuits  by  means  of  cutting  lines 
of  force  are  said  to  be  caused  by  Electromagnetic  Induction,  and  they 
are  sometimes  spoken  of  as  Induction  Currents  or  Induced  Currents. 
The  currents  produced  by  dynamos  are  examples  of  currents  induced 
by  electromagnetic  action.  When  the  coils  act  directly  upon  each 
other,  the  effect  is  called  Mutual  Induction. 

139.  Induction   Coils.  —  An  appliance   consisting  of  a  primary  coil 
and  a  secondary  coil,  which  is  used  for  the  purpose  of  inducing  currents 
in  the  circuit  of  the  secondary  coil  by  varying  the  current  in  the  pri- 
mary coil,  is  called  an  Induction  Coil.    The  two  windings  of  an  induction 
coil  are  usually  placed  on  an  iron  core,  which  greatly  increases  their 
effectiveness.     The  core  must  be  made  of  iron  wires,  or  currents  will  be 
induced  in  the  core  and  thus  heating  and  loss  of  power  will  result,  since 
currents  are  induced  in  all  closed  circuits  or  masses  of  metal  which  are 
in  a  changing  magnetic  field.     This  division  of  the  iron  core  of  an  induc- 
tion coil  is  necessary  for  the  same  reason  that  it  is  necessary  to  laminate 
the  iron  cores  of  dynamo  armatures,  as  will  be  described  in  a  later  chapter. 


ELECTRICITY   AND    MAGNETISM 


Each  turn  of  the  secondary  windings  of  a  well-built  induction  coil 
cuts  practically  all  of  the  lines  of  force  which  are  set  up  by  the  current 
in  the  primary  coil,  so  that  the  total  electrical  pressure  induced  in 

the  secondary  wind- 
ings may  be  con- 
trolled by  winding 
the  secondary  coil 
with  a  greater  or 
less  number  of  turns 
of  wire.  In  the  in- 
duction coils  made 
for  scientific  experi- 
ments, which  are  of- 
ten called  Ruhmkorff 
coils  (Fig.  75),  the 


FIG.  75.  —  Ruhmkorff  Coil. 


SECONDARY 


secondary      has      so 
very   many    turns   of 

extremely  fine  wire  that  the  pressure  produced  in  the  secondary,  when 
the  current  from  a  few  battery  cells  is  made  and  broken  in  the  primary 
coil,  may  be  so  great  as  to  cause 
an  electric  spark  to  jump  a  num- 
ber of  inches  through  air.  In  the 
induction  coils  commonly  called 
Transformers  or  Converters  (Fig. 
76),  which  are  common  objects 
on  the  poles  of  electric  light  com- 
panies which  use  alternating  cur- 
rents, the  secondary  coils  usually 
have  fewer  turns  than  the  primary 
coils,  and  the  electrical  pressure 
induced  in  the  secondary  coils  is 
therefore  less  than  the  pressure  ap- 
plied to  the  primary.  Commercial  FIG.  76.  — Alternating  Current  Transformer. 
transformers  are  usually  enclosed 

in  iron  cases,  but  the  illustration  (Fig.  76)  simply  shows  the  coils  and 
iron  core  which  compose  the  essential  parts  of  the  device.     Such  trans- 


ELECTROMAGNETIC   INDUCTION  151 

formers  are  used  to  reduce  a  high  pressure  which  is  used  on  the  street 
circuits  to  a  lower  pressure  which  may  safely  and  conveniently  be  used 
in  buildings  to  operate  electric  lights.  Transformers,  as  applied  to  elec- 
tric lighting,  will  receive  attention  in  a  later  chapter.  By  means  of  them 
we  are  able  to  perform  the  remarkable  feat  of  commercially  transferring 
electrical  power  from  one  circuit  to  another,  although  the  circuits  have 
absolutely  no  electrical  connection  with  each  other. 

140.  General  Rule  for  the  Direction  of  Induced  Pressures. —  If  we  re- 
member the  direction  of  the  lines  of  force  around  a  wire  which  carries 
a  current1  and  the  rule  for  determining  the  direction  of  an  induced  cur- 
rent,2 it  is  easy  to  determine  the  direction  of  the  current  induced  in  any 
secondary  circuit.     By  applying  the  rules  referred  to,  the  following  rules 
relating  to  induced  currents  may  be  derived  :  — 

1.  When  a  primary  coil  is  PUSHED  INTO  a  secondary  coil,  the  secondary 
induced  current  is  OPPOSITE  IN  DIRECTION  to  the  primary  current. 

2.  When  a  primary  coil  is  DRAWN  OUT  of  a  secondary  coil,  the  induced 
secondary  current  is  in  the  SAME  DIRECTION  as  the  primary  current. 

When  the  primary  and  secondary  coils  are  fastened  together  and  cur- 
rent is  induced  in  the  secondary  by  making  and  breaking  the  primary 
current,  we  have  the  following  rules  :  — 

3.  When  the  current  is  MADE  (started]  in  the  primary  coil,  a  momen- 
tary OPPOSITE  or  INVERSE  current  is  induced  in  the  secondary  coil. 

4.  When  a  current  is  BROKEN  (stopped)  in  the  primary  coil,  a  momen- 
tary current  of  the  SAME  DIRECTION  is  induced  in  the  secondary  coil. 

These  rules  relate  to  the-  flow  of  current  when  the  secondary  circuit  is 
closed.  If  the  secondary  circuit  is  open,  the  electrical  pressure  which  is 
set  up  is  in  such  a  direction  that  the  current  would  flow  in  the  direction 
indicated  were  the  circuit  closed. 

141.  Lenz's  Law.  —  A  careful  examination  of  these  rules  shows  a 
very  important  fact  which  may  be  stated  in  this  way  :   The  direction  of 
an  induced  current  is  always  such  that  the  magnetic  field  belonging  to  it 
tends  to  oppose  the  change  in  the  strength  of  the  magnetic  field  belonging 
to  the  primary  current.     For  instance,  when  the  primary  current  of  an 

1  Article  122.  2  Article  135. 


152  ELECTRICITY   AND    MAGNETISM 

induction  coil  is  "  made,"  an  inverse  current  is  induced  in  the  secondary 
coil  whose  magnetic  field  opposes  the  growth  of  the  magnetic  field  of 
the  primary  current.  When  the  primary  circuit  is  broken,  the  magnetic 
field  of  the  induced  current  opposes  the  decay  of  the  magnetic  field  be- 
longing to  the  primary  current.  Another  illustration  may  be  taken  from 
the  primary  coil  which  is  pushed  into  a  secondary  coil.  When  the 
primary  coil  carrying  a  current  is  pushed  into  the  secondary,  an  inverse 
current  is  induced  which  sets  up  a  magnetic  field  which  tends  to  repel 
the  primary  coil  and,  therefore,  opposes  its  motion.  When  the  primary 
coil  is  drawn  out  of  the  secondary,  the  direct  induced  current  sets  up  a 
magnetic  field  which  tends  to  attract  the  primary  coil  and,  therefore, 
again  opposes  its  motion. 

In  the  case  of  a  dynamo,  the  current  which  is  induced  in  the  armature 
conductors  has  such  a  direction  that  its  magnetic  effect  tends  to  stop  the 
motion  of  the  armature  ;  and  to  keep  it  rotating,  mechanical  power  must 
be  applied  to  the  armature  in  proportion  to  the  amount  of  power  repre- 
sented by  the  currents  generated. 

The  above  facts  may  be  briefly  stated  in  one  sentence.  When  electric 
currents  are  induced  by  a  changing  magnetic  field,  the  magnetic  field  be- 
longing to  the  induced  currents  tends  to  stop  the  change  in  the  original 
field.  We  have  also  the  following  statement  which  results  directly  from 
the  former.  }VJien  electric  currents  are  induced  by  the  motion  of  a  con- 
ductor, tlie  induced  currents  have  such  a  direction  that  their  magnetic 
effect  tends  to  stop  the  motion.  This  is  called  Lenz's  Law,  after  a  German 
scientist  who  first  formally  stated  the  principle. 

The  principles  stated  in  the  preceding  paragraph  are  a  direct  result 
of  the  general  law  of  the  Conservation  of  Energy.1  We  can  transform 
mechanical  energy  into  electrical  energy  or  vice  versa  ;  or,  we  can  trans- 
form the  energy  of  electrical  currents  flowing  under  one  pressure  into 
the  energy  of  electrical  currents  flowing  under  another  pressure ;  but  in 
every  case  as  much  energy  must  be  put  into  the  transforming  apparatus 
—  whether  it  be  dynamo,  motor,  Ruhmkorff  coil,  or  transformer  —  as  is 
taken  out.  We  have  already  seen  that  the  useful  "output  "  of  electrical 
apparatus  is  usually  smaller  than  the  "  input  "  by  a  certain  percentage  of 
the  total  energy  which  has  been  changed  into  useless  heat. 

1  Article  in. 


ELECTROMAGNETIC   INDUCTION  153 


PROBLEMS 

A.  A  coil,  containing  an  iron  core,  has  $00,000  lines  passing  through  it.     When 
this  coil  is  pushed  into  another  coil  of  100  turns  at  a  fixed  speed  an  average  pressure 
of  5  volts  is  induced.     How  many  turns  must  the  secondary  coil  have  in  order  that 
the  induced  pressure  may  be  100  volts?     Ans.  2000  turns. 

B.  In  an  induction  coil  having  2000  turns  on  the  secondary  the  average  alternat- 
ing pressure  induced  is  500  volts.    How  many  turns  must  the  secondary  have  in  order 
that  100,  1000,  and  10,000  volts  may  be  induced?     Ans.  400,  4000,  40,000  turns. 

C.  If  work  equal  to  30  watts  for  one  second  is  transferred  from  a  primary  coil  to 
a  secondary  coil  while  the  former  is  being  pushed  into  the  latter  as  described  in 
Example  A,  how   many  foot   pounds   of   work  must  be   expended   in  pushing  the 
primary  coil,  provided  that  no  appreciable  losses  occur  in  the  transformation  ?    (Aid : 
Apply  the  law  of  the  conservation  of  energy.)     Ans.    22.1  (approx.). 

D.  If  100  watts  are  obtained  from  the  secondary  coil  of  a  transformer,  how  many 
watts  must  be  put  into  the  primary,   supposing  the  losses  of  transformation  equal 
10  per  cent  of  the  input  ?     Ans.   in  watts  (approx.). 

142.  Self-induced  Pressures.  —  A  varying  current  may  have  an  in- 
ductive effect  upon  the  coil  in  which  it  flows  itself,  in  addition  to  its 
inductive  effect  upon  adjacent  conductors.  When  a  current  is  started 
in  a  coil  it  sets  up  a  magnetic  field  which  quickly  grows  from  zero  to  its 
full  value.  As  the  field  grows,  its  lines  of  force  cut  the  turns  of  the  coil 
and  induce  in  them  an  electric  pressure  which  opposes  the  growth  of  the 
current.  On  stopping  the  original  current,  its  magnetic  field  quickly 
dies  away  and  the  lines  of  force  again  cut  the  turns  of  the  coil,  but  this 
time  in  such  a  direction  that  the  self-induced  electric  pressure  upholds 
the  original  current.  If  the  coil  has  a  great  many  turns  wound  on  an 
iron  core,  its  Self-induction  may  be  of  sufficient  magnitude  to  make  a 
brilliant  spark  or  give  a  severe  shock  when  the  circuit  is  broken.  The 
spark  at  breaking  a  circuit  is  often  spoken  of  as  caused  by  the  extra 
current  of  self-induction.  The  effect  of  self-induction  is  made  use  of  in 
so-called  Spark  Coils,  which  are  used  with  devices  for  lighting  gas  by 
electricity,  and  which  consist  simply  of  a  coil  containing  many  turns  of 
insulated  wire  wound  on  a  core  of  iron  wire.  The  effect  of  self-induction 
makes  itself  evident  if  the  circuit  of  a  single  battery  cell  is  broken 
between  the  hands  when  the  circuit  contains  a  spark  coil,  telegraph 
instrument,  or  other  electromagnetic  coil. 


154 


ELECTRICITY   AND    MAGNETISM 


143.  Mutual  Attraction  or  Repulsion  of  Electric  Circuits.  —  The 
fact  that  a  conductor  carrying  an  electric  current  is  always  surrounded 
by  a  magnetic  field 1  would  lead  us  to  expect  conductors  carrying  electric 
currents  to  attract  and  repel  each  other.  This  is  indeed  the  fact.  We 
have  already  seen  that  solenoids  act  towards  each  other  exactly  as 
though  they  were  magnets.2  In  every  case  we  have 
learned  that,  where  magnets  or  solenoids  are  brought 
into  each  other's  influence,  they  tend  to  move  so  that 
their  lines  of  force  shall  be  placed  parallel  and  in  the 
same  direction.  Exactly  the  same  is  true  of  straight 
or  curved  wires  which  are  brought  into  each  other's 
influence.  Remembering  this,  we  can  see  that  two 
wires  lying  side  by  side  must  attract  each  other  if  they 
carry  currents  flowing  in  the  same  direction.  This  is 
because  the  lines  of  force  can  only  become  parallel 
and  of  the  same  direction  when  the  two  conductors 
are  very  close  together.  When  the  currents  flow  in 
opposite  directions  the  wires  repel  each  other.  In  the 
same  way,  if  the  wires  are  inclined  to  each  other  they 
tend  to  turn  around  into  such  a  position  that  the  wires  are  parallel  and 
the  currents  flow  in  the  same  direction  (Fig.  77).  This  principle  is 
used  in  the  design  of  electrical  measuring  instruments,  such  as  are 
described  amongst  others  in  Chapters  XI  and  XIII. 


FIG.  77.  —  Parallel 
Wires  carrying 
Currents. 


QUESTIONS 

38.  How  can  an  induced  pressure  be  set  up  without  any  mechanical  movement? 

39.  When  the  current  is  made  and  broken  in  a  primary  coil,  what  are  the  relative 
directions  of  the  currents  induced  in  the  secondary? 

40.  If  the  current  has  a  steady  value  in  the  primary  coil,  will  a  pressure  be  induced 
in  the  secondary? 

41.  What  is  mutual  induction? 

42.  What  are  induced  currents  or  pressures  called? 

43.  What  is  an  induction  coil? 

44.  Why  is  an  iron  core  used  in  an  induction  coil? 

45.  Why  is  the  core  of  an  induction  coil  not  made  of  solid  iron? 


1  Chapter  IX. 


2  Article  124. 


ELECTROMAGNETIC  INDUCTION  155 

46.  What  is  a  Ruhmkorff  coil? 

47.  What  is  a  transformer? 

48.  How  can  a  high  or  low  pressure  be  obtained  by  means  of  induction  coils? 

49.  Give  the  two  rules  showing  the  relative  directions  of  the  induced  and  inducing 
currents  in  movable  primary  and  secondary  coils. 

50.  Give  the  rules  as  in  Question  49  when  the  two  coils  are  fixed. 

51.  If  a  primary  coil  carrying  a  current  is  pushed  into  the  secondary,  will  the 
induced  current  be  in  the  same  direction  as  if  the  coil  was  first  pushed  in  and  the 
primary  current  was  then  started  ? 

52.  What  is  Lenz's  Law  ? 

53.  Illustrate  the  application  of  Lenz's  Law  to  a  pair  of  movable  and  a  pair  of 
mutually  fixed  induction  coils. 

54.  If  a  conductor  forming  part  of  a  circuit  is  taken  in  the  hand  and  cut  rapidly 
across  the  field  of  a  strong  electromagnet,  what  will  happen?     Why? 

55.  In  what  way  does  Lenz's  Law  follow  from  the  law  of  the  conservation  of 
energy? 

56.  Will  more  energy  be  put  into  the  primary  coil  of  an  induction  coil  when  the 
secondary  is  closed  than  when  it  is  open?     Why? 

57.  Does  it  take  more  work  to  thrust  a  coil  carrying  a  current  into  another  when 
the  latter  is  connected  to  a  closed  circuit,  than  if  its  circuit  is  open  ? 

58.  What  is  a  self-induced  pressure? 

59.  Why  is  there  an  extra  current  of  self-induction  when  a  circuit  is  broken? 

60.  What  is  a  spark  coil? 

61.  When  a  current  is  building  up,  does  the  self-inducted  pressure  aid  or  impede 
the  original  current?     How  is  it  when  a  circuit  is  broken? 

62.  Does  Lenz's  Law  apply  to  cases  of  self-induction? 

63.  Do  currents  which  flow  in  the  same  direction  in  adjacent  wires  repel  each 
other  ?     Do  they  attract  each  other  ? 

64.  Do  currents  which  flow  in  opposite  directions  in  adjacent  wires  repel  each 
other  ? 


CHAPTER   XI 

GALVANOMETERS   AND   VOLTAMETERS 

144.  Galvanometers.  —  Instruments  for  detecting  and  measuring  elec- 
tric currents,  the  indications  of  which  are  dependent  upon  the  deflection 
of  a  magnetic  needle  caused  by  the  magnetic  effect  of  the  current 
flowing  in  a  coil  which  surrounds  the  needle,  are  called  Galvanometers. 
These  instruments  are  made  in  a  great  variety  of  forms,  and  are  widely 
used  for  measurements  in  shops  and  laboratories. 

In  most  forms  of  galvanometers  the  magnetic  needle  is  placed  at  the 
centre  of  a  coil  of  wire.  This  coil  may  have  a  great  number  of  turns  of 
fine  wire,  in  which  case  the  galvanometer  is  "sensitive,"  —  that  is,  the 
needle  is  appreciably  deflected  by  a  very  small  current ;  or  the  coil  may 
have  but  few  turns  of  thick  wire,  in  which  case  the  galvanometer  is  in- 
tended for  use  with  comparatively  large  currents.  In  some  cases  the 
coil  of  the  galvanometer  is  placed  so  that  it  stands  in  an  exact  north 
and  south  position  (that  is,  in  the  magnetic  meridian)  like  the  needle. 
The  magnetic  force  due  to  the  coil,  which  is  at  right  angles  to  its  wire,1 
is  then  at  right  angles  to  the  magnetic  force  of  the  earth  and  also  to  the 
length  of  the  needle.  When  the  coil  is  in  this  position,  a  current  in  the 
coil  exerts  its  greatest  force  to  deflect  the  needle. 

When  a  galvanometer  is  connected  in  a  circuit,  the  presence  of  a  cur- 
rent is  shown  by  the  deflection  of  the  needle.  The  direction  of  the 
current  is  shown  by  the  side  toward  which  the  north  pole  of  the  needle 
moves.2  The  strength  of  the  current  is  indicated  by  the  amount  of  the 
needle's  deflection,  since  the  position  which  the  needle  takes  depends 
upon  the  relative  magnitude  of  the  magnetic  forces  due  to  the  current 
and  the  earth."  The  earth's  magnetism  may  be  considered  to  be  ap- 
proximately constant  at  any  fixed  position. 

1  Article  1 21.  2  Article  122.  3  Article  120. 

156 


GALVANOMETERS   AND   VOLTAMETERS 


157 


145.  Tangent  Galvanometer.  —  When  the  diameter  of  the  galvanom- 
eter coil  is  very'  great  compared  with  the  length  of  the  needle,  the  trigo- 
nometrical tangents  of  the  angles  through  which  the  needle  is  deflected  by 
various  currents  are  proportional  to  the  currents.  Such  a  galvanometer 
is  called  a  Tangent  Galvanometer  (Fig. 
78).  -Other  galvanometers  in  which  the 
coil  is  moved  so  as  to  bring  the  needle 
back  to  zero  (Fig.  79),  are  called  Sine 


FiG.  78.  —  Tangent  Galvanometer. 


FIG.  79.  —  Sine  Galvanometer. 


Galvanometers,  because  the  trigonometrical  sine  of  the  angle  through 
which  the  coil  is  moved  is  proportional  to  the  current  causing  the 
deflection.  In  some  rough  galvanometers  a  pointer  is  attached  to  the 
needle,  and  the  deflection  is  read  off 'on  a  divided  circle,  over  which 
the  pointer  moves.  The  circle  of  such  a  galvanometer  is  usually 
divided  uniformly  in  degrees. 

146.  Reflecting  Galvanometers.  —  For  exact  measurements,  so  rough 
a  method  of  reading  deflections  is  not  sufficiently  accurate,  and  Reflecting 
Galvanometers  are  used  (Fig.  80).  A  small  mirror  is  attached  to  the 
magnet  in  these,  and  the  deflections  are  read  off  by  means  of  a  small 
telescope  through  which  a  reflection  of  a  stationary  scale  is  seen  in  the 
mirror.  When  the  needle  moves,  the  mirror  moves  with  it,  and  the  reflec- 
tion of  the  scale  as  seen  in  the  telescope  appears  also  to  move  and  the 
deflection  of  the  needle  is  thus  determined.  Instead  of  using  a  telescope 
and  scale,  as  is  usually  done  in  America,  a  lamp  and  scale  (Fig.  81), 
may  be  used.  In  this  case  a  beam  of  light  from  a  lamp  which  is  placed 


ELECTRICITY   AND   MAGNETISM 


behind  a  slit  in  front  of  the  galvanometer  is  reflected  by  the  mirror  upon 
a  scale,  where  it  shows  as  a  spot  of  light.     When  the  needle  with  its 


FlG.  80.  —  Reflecting   Galvanom 
eter. 


FIG.  81.  — Lamp  Stand  and  Scale. 

mirror  is  deflected,  the  spot  of  light  moves 
along  the  scale,  thus  showing  the  magni- 
tude of  the  deflection.  This  is  a  very 
convenient  arrangement  to  use  when  test- 
ing must  be  done  in  dark  rooms  or  vaults, 
but  it  cannot  be  used  in  a  light  place. 

147.  Needle  Supports.  —  The  support 
of  the  needle  is  sometimes  on  a  finely 
wrought  pivot,  and  the  needle  is  then 
set  with  an  agate  or  ruby  centre  so  that 
it  may  move  easily.  The  friction  of  the 

finest  pivot,  however,  is  so  great  that  it  destroys  the  sensitiveness  of  a 
fine  galvanometer,  so  that  in  all  fine  galvanometers  the  needles  are  sus- 
pended by  means  of  a  Fibre  which  is  usually  made  of  jinspitn  cocoon 
silk.  This  fibre  must  be  strong  enough  to  support  the  weight  of  the 
needle  with  its  mirror,  but  it  should  be  as  fine  as  possible,  so  that  it  will 
not  oppose  the  least  force  against  the  deflection  of  the  needle.  The 
needle  and  mirror  are  usually  made  as  light  as  possible  and  the  suspend- 
ing fibre  is  sometimes  so  fine  that  it  can  scarcely  be  seen.  The  length 
of  the  Suspension  varies  from  a  small  fraction  of  an  inch  to  many  inches. 
Fibres  are  now  often  made  from  very  fine  hairs  of  quartz,  which  has 
been  melted  in  a  blowpipe  flame  and  drawn  out. 


GALVANOMETERS   AND   VOLTAMETERS 


159 


148.  Controlling  Magnets.  —  It  is  often  convenient  to  make  the  needle 
of  a  galvanometer,  independent  of  the  direction  of  the  earth's  magnet- 
ism or  to  vary  the  strength  of  the  Directive   Force,  that  is,  the  force 
which  holds  the  needle  in  the  magnetic  meridian.     For  this  purpose 
galvanometers  are  generally  arranged  with  one  or  more  Directive  Mag- 
nets or  Controlling  Magnets.     One  is  shown  as  a  curved  bar  placed  on 
a  stem  above  the  galvanometer  of  Figure  80.    By  varying  the  position  of 
the  magnet  with  respect  to  the  needle,  the  position  of  rest  taken  by  the 
needle  may  be  controlled  as  desired,  and  the  galvanometer  may  be  set 
in  any  desired  position. 

149.  Astatic  Needles.  —  In  order  that  a  galvanometer  may  be  made 
very  sensitive,  it  is  desirable  to  make  the  controlling  force  very  weak,  — 
in  some   cases  much  weaker  than  that 

due  to  the  earth's  magnetism.  Conse- 
quently the  effect  of  the  earth's  mag- 
netism must  be  overcome.  For  this 
purpose  what  are  known  as  Astatic 
Needles  are  used.  These  consist  of  a 
pair  of  needles  of  practically  equal  size 
and  magnetic  strength  which  are  fas- 
tened to  a  light  thin  wire,  one  above 
the  other,  so  that  their  north  poles 
point  in  exactly  opposite  directions.  It 
is  usual  to  arrange  a  coil  of  wire  for 
each  needle,  so  that  the  galvanometer 
has  two  coils,  and  the  mirror  may  be 
fastened  to  any  convenient  part  of  the 
supporting  wire  (Fig.  82).  In  some 
very  sensitive  galvanometers  there  are 
eight  needles  'arranged  Astatically,  and 
eight  coils. 

150.  Forms  of  Needles.  —  The  forms 
in  which  galvanometer  needles  are  made 
are  quite  various.     Some  needles  are  in 

the  form  of  a  partially  split  bell,  one  side  being  the  north  pole  and  the 
other  being  the  south  pole.     Other  needles  are  made  of  flat  disks  or 


''  * 


i6o 


ELECTRICITY  AND   MAGNETISM 


FlG.  820.  —  Galvanometer 
Needle,  composed  of 
Several  Small  Mag- 
nets cemented  to  a 
Disk. 


rings  which  are  so  magnetized  that  a  portion  of  the  edge  serves  as  the 

north  pole  and  the  opposite  portion  as  the  south  pole.  The  commonest 
form  of  needle  is  one  built  up  of  several  little 
magnets,  made  from  a  watch  spring,  which  are 
laid  side  by  side  with  the  poles  all  the  same  way. 
These  are  usually  fastened  to  the  back  of  the 
galvanometer  mirror  or  to  a  little  disk  of  alumi- 
num, as  shown  in  Fig.  82^,  where  c,  <:,  c,  are  the 
little  magnets  and  A  is  the  suspension. 

151.  D'Arsonval  Galvanometer.  —  A  very  con- 
venient form  of  galvanometer  is  one  in  which 
the  coil  is  suspended  so  as  to  move  in  the 
magnetic  field  of  a  strong  horseshoe  magnet 

(Fig.  83).     In  this  instrument   the   relations  of  coil  and   magnet  are 

practically  the  reverse  of  those  in  the  common  galvanometers.     This 

is  called  a  D'Arsonval  Galvanometer, 

after  a  French    scientist  who    put   it 

into  useful  form.     The  suspension  of 

the  coil  of  a  D'Arsonval  galvanometer 

must  be  arranged  so  that  the  current 

may  get  into  and  out  of  the  coil.     The 

coil  is,  therefore,  often  supported  be- 
tween stretched  phosphor-bronze  wires 

which  are  connected  to  it  at  the  top 

and  bottom  and  which  serve  as  leads 

for  the  current.     Sometimes  the  coil 

is   suspended   on    a    silver    wire    by 

means  of  which  the  current  can  enter 

the  coil,  and    a  wire   at   the   bottom 

of  the  coil  dips  into  a  bottle  of  mer-    ] 

cury    so    that    the    current    can    get 

out. 

152.    Dead-beat  Galvanometer.  —  One  reason  that  a  D'Arsonval  gal- 
vanometer is  convenient   for   general  use  is,  because  it  is  Dead-beat; 

that  is,  when  the  coil  is  deflected  it  goes  at  once  to  its  position  without 

a  tedious  period  of  swinging  back  and  forth.     Ordinary  galvanometers 


" 

83.  —  D'Arsonval  Galvanometer. 
M,  mirror;  C,  coil;  A7,  magnet; 
D,  stationary  soft  iron  core. 


GALVANOMETERS   AND    VOLTAMETERS 


161 


may  be  made  more  or  less  dead-beat  by  surrounding  the  needle  with  a 
ball  of  copper,  or  by  attaching  to  the  suspension  fibre  small  wings  of 
mica  or  aluminum,  which  are  enclosed  in  a  small  chamber. 

153.  Galvanometer  Constant." —  In  order  that  a  galvanometer  may  be 
used  to  actually  measure  electric  currents  in  amperes,  the  Constant  of 
the  Galvanometer  must  be  known,  or  the  galvanometer  must  be  Calibrated 
or  Standardized. 

When  the  deflections  of  a  galvanometer  bear  some  fixed  relation  to 
the  currents  causing  the  deflections,  it  is  said  to  have  a  Constant.  For 
instance,  in  the  case  of  a  tangent  galvanometer,  the  current  which 
causes  a  certain  deflection  of  the  needle  is  given  in  amperes  by  multi- 
plying the  tangent  of  the  angle  of  deflection  by  the  constant  of  the 
galvanometer.  The  constant  of  a  tangent  galvanometer  may  be  directly 
calculated  when  the  coil  is  circular,  and  its  diameter  and  number  of 
turns  and  the  strength  of  the  earth's  magnetism  are  known.  The  con- 
stant may  also  be  determined  by  passing  a  current  of  known  strength 
through  the  galvanometer  and  observing  the  deflection.  The  needle 
of  a  reflecting  galvanometer  moves  through  so  small  an  arc  that  the 
deflection  may  frequently  be  taken  to  be  proportional  to  the  current ; 
this  is  almost  always  the  case  with  D'Arsonval  galvanometers.  The 
constant,  under  such  circumstances,  is  the  amount  of  current  required 
to  cause  a  deflection  of  one 
scale  division.  The  constant 
is  also  sometimes  defined  as 

CO 

the  number  of  divisions  of  the  2 
deflection  caused  by  a  certain  g 
battery  when  acting  through  a  ! 
resistance  of  one  megohm. 

When  the  deflections  of  the  I 
galvanometer  are  not  known  < 
to  bear  a  fixed  relation  to 
the  currents  causing  them,  the 
galvanometer  must  be  experi- 
mentally calibrated.  That  is, 
currents  of  various  known  strengths  must  be  passed  through  the  galva- 
nometer and  the  deflections  observed.  These  observations  may  be  set 
M 


.0001 


.0004 


.0002  .0003 

AMPERES 

FlG.  84.  —  Calibration  Curve  of  a  Galvanometer. 


1 62 


ELECTRICITY  AND    MAGNETISM 


down  in  a  table,  so  as  to  be  used  in  future  work  with  the  galvanometer, 
or  the  observations  may  be  platted  in  a  curve  on  cross-ruled  paper. 
Such  a  Calibration  Curve  is  often  convenient,  since  the  value  of  a  cur- 
rent corresponding  to  any  deflection  may  be  at  once  determined  from 
it.  A  calibration  curve  is  illustrated  in  Figure  84. 

When  galvanometers  are  used  simply  for  the  detection  of  currents,  or 
for  comparing  the  relative  magnitudes  of  currents,  as  is  frequently  the  case, 
calibration  is  unnecessary. 

PROBLEMS 

A.  A  deflection  of  200  divisions  is  caused  by  sending  .001  of  an  ampere  through 
a  reflecting  galvanometer.     If  the  deflections  are  proportional  to  the  current,  what  is 
the  galvanometer  constant?     Ans.  .000005  of  an  ampere. 

B.  A  reflecting  galvanometer  and  resistance  box  in  series  have  a  total  resistance 
of  100,000  ohms.     One  volt  pressure  causes  a  deflection  of  100  divisions  on  the  gal- 
vanometer scale.     If  the  deflections  are  proportional  to  the  current,  what  is  the  con- 
stant of  the  galvanometer?     Ans.  .0000001  of  an  ampere. 

C.  A  certain  current  causes  a  deflection  of  100  divisions  on  a  galvanometer  scale. 
If  the  constant  of  the  galvanometer  is  .0001,  what  is  the  current?     Ans.  .01  of  an 

ampere. 

D.  The  total  resistance  of  a  circuit  containing  a 
galvanometer  is  1000  ohms.  The  constant  of  the  gal- 
vanometer is  .0001.  If  the  galvanometer  is  deflected 
loo  divisions,  what  pressure  is  impressed  upon  the 
terminals  of  the  circuit?  Ans.  10  volts. 


154.  Galvanometer  Shunts.  —  It  is  often  de- 
sirable to  be  able  to  use  a  sensitive  galvanom- 
eter for  reasonably  rough  measurements,  and 
thus  avoid  the  duplication  of  expensive  instru- 
ments. For  this  purpose  the  galvanometer 
may  be  Shunted,1  by  connecting  any  desired 
resistance  across  the  terminals  (Fig.  85)  in 
such  a  way  that  the  current  of  the  circuit  will 
divide  between  the  galvanometer  coils  and  the 
Shunt.  As  a  rule,  it  is  desired  to  make  this 
division  of  current  in  some  even  ratio,  as  i  :  9, 
i  :  99,  i  :  999,  etc.,  and  then  the  resistance 


-99- 


FIG.  85.  —  Galvanometer  and 
Shunt. 


1  Article  105. 


GALVANOMETERS  AND  VOLTAMETERS 


I63 


of  the  shunt  must  bear  the  proper  ratio  to  the  resistance  of  the  galva- 
nometer windings.  Suppose  it  is  desired  to  make  a  shunt  of  such 
resistance  that  its  use  will  cause  exactly  one-tenth  of  the  total  current  to 
flow  through  the  galvanometer,  then  nine-tenths  of  the  current  will  flow 
through  the  shunt.  By  the  laws  of  divided  circuits  *  the  resistance  of 
the  shunt  must  be  one-ninth  of  the  resistance  of  the  galvanometer. 
When  the  resistance  of  the  shunt  is  one  ninety-ninth  of  the  resistance 
of  the  galvanometer,  one-hundredth  of  the  current  will  flow  through 
the  latter;  and  when  the  shunt  resistance  is  one  nine-hundred-ninety- 
ninth  (^5-9)  of  the  galvanometer  resistance,  one-thousandth  of  the  cur- 
rent will  flow  through  the  latter. 

Galvanometers  usually  have  corresponding  shunt  boxes  sold  with  them, 
which  have  three  coils,  respectively  marked  ^,  -^,  and  9-^-9.  When  the 
shunt  box  is  connected  in  parallel  with  the 
galvanometer,  either  of  these  shunts  may  be 
placed  in  the  circuit  by  means  of  a  plug,  or 
the  shunt  circuit  may  be  entirely  broken. 
When  a  shunt  is  plugged  into  the  circuit, 

ro>  yio>  or  ToW  Part  of  the  whole  current 
flows  through  the  galvanometer,  depending 
upon  the  position  of  the  plug.  Figure  86 
shows  a  common  form  of  shunt  box,  and 

Figure  8c;  shows  the  connections  of  the  gal- 

FiG.  86.  —  Exterior  of  Shunt  Box. 
vanometer  and  shunt  box.     When  the   plug 

is  inserted  in  either  of  the  holes  marked  C,  the  corresponding  shunt 
is  connected  in  the  circuit. 

PROBLEMS 

A.  A  galvanometer  has  a  resistance  of  1200  ohms;   it  is  desired  to  shunt  it  so 
that  only  .001  of  the  total  current  to  be  measured  will  pass  through  it.     What  must 
be  the  resistance  of  the  shunt?     Am.  1.2012  ohms  (approx.). 

B.  A  galvanometer,  in  which  the  deflections  are  proportional  to  the  current,  is 
deflected  100  scale  divisions  by  I  milliampere.     What  is  the  strength  of  a  current 
which  causes  a  deflection  of  150  divisions,  when  the  galvanometer  is  shunted  by  a  fa 
shunt?     Ans.   150  milliamperes. 

C.  A  galvanometer,  having  a  constant  of  .00005  °f  an  amPere  Per  division,  shows 
a  reading  of   200  divisions  when  shunted  by  a  •$%•$  shunt  box.      What   current    is 
flowing?     Ans.  10  amperes. 

1  Article  103. 


164  ELECTRICITY   AND   MAGNETISM 

D.  A  deflection  of  20  divisions  is  caused  by  sending  .001  of  an  ampere  through 
a  reflecting  galvanometer,  which  is  shunted  by  a  \  box.  If  the  deflections  are  pro- 
portional to  the  current,  what  is  the  galvanometer  constant?  Ans.  .000005  °f  an 
ampere. 

155.  Voltameter.  —  An  instrument  for  measuring  currents  by  means 
of  their  electrochemical  action,  which  is  often  used  in  calibrating  gal- 
vanometers, is  called  a  Voltameter.     We  have  already  seen  that  chemi- 
cal action  goes  on  in  a  battery  cell  or  electrolytic  vat,1  when  a  current  is 
passed  in  either  direction  through  the  cell,  and  that  the  amount  of  the 
action  is  proportional  to  the  number  of  coulombs  of  electricity  passed 
through  the  cell.     The  chemical  action  in  a  voltameter  is  similar  to  that 
which  takes  place  in  a  voltaic  cell,  but  both  plates  are  of  the  same 
material,  and  there  is,  therefore,  no  tendency  to  set  up  a  current  due  to 
the  direct  action  of  the  cell.2 

156.  Water  Voltameter.  —  The  earliest  form  of  voltameter  is  one  in 
which  sulphuric  acid,  greatly  diluted  by  water,  is  electrolyzed.3     This  is 
called  a  Water  Voltameter.     A  form  of  water  voltameter  is  shown  in 
Figure  30.     When  this  is   to  be  used,  diluted  acid  is  poured  into  the 
funnel  C,  at  the  back,  and  rises  to  the  top  of  the  two  arms,  AB,  in 
front,  if  the  stopcocks  at  their  tops  are  open.     After  the  tubes  are  filled 
the  cocks  are  closed,  and  the  current  is  passed  between  the  platinum 
electrodes,  EE.      The   electrochemical  action  set  up  by  the  current 
causes  oxygen  to  go  to  the  positive  pole  or  anode,  and  hydrogen  to  go 
to  the  negative  pole  or  cathode.     The  gases  rise  in  the   tubes  above 
their  respective  electrodes  arid  displace  the  water.     The  direct  action 
of  the  current  causes  a  decomposition  of  the  sulphuric  acid  in  the  water, 
but  additional  chemical  action  is  set  up  by  the  decomposed  sulphuric 
acid,  which  makes  the  total  action  equivalent  to  the  decomposition  of 
water. 

Water  is  composed  by  bulk  of  two  parts  of  hydrogen  to  one  part  of 
oxygen,  and  consequently  the  tube  over  the  cathode  collects  twice  as 
much  gas  as  that  over  the  anode.4  If  a  steady  current  is  passed  through 
such  a  voltameter  for  a  given  number  of  seconds,  the  strength  of  the 
current  can  be  determined  from  the  amount  of  gases  collected  per 

1  Chapters  IV  and  V.  3  See  Articles  61  and  66. 

2  Chapter  V.  •*  Article  66. 


GALVANOMETERS   AND    VOLTAMETERS  1 65 

second.  For,  the  number  of  coulombs  of  electricity  passed  through  the 
voltameter  is  determined  from  the  amount  of  the  gases  collected  and 
their  electrochemical  equivalent.1  The  number  of  coulombs  passed 
through  the  circuit  per  second  is  equal  to  the  current  in  amperes. 

157.  Metal  Voltameter.  —  A  water  voltameter   is  not   a  very   con- 
venient or  satisfactory  instrument,  and  voltameters  in  which  the  electro- 
lytes  are   solutions   of   the   salts    of  metals   are   preferred   for  actual 
measurements.     When  such  a  solution  is  Electrolyzed  between  plates 
of  the  metal  contained  in  the  solution,  the  solution  is  decomposed  ; 
the  metal  from  the  solution  goes  with  the  current  to  the  cathode  where 
it  is  deposited,  and  the  acid  part  of  the  compound  goes  to  the  anode, 
which  it  attacks,  and  with  the  metal  forms  a  new  portion  of  the  com- 
pound,   which   is   dissolved    in   the    solution.       The    cathode    should, 
therefore,  be  expected  to  gain  exactly  as  much  metal  from  the  deposit 
as  the  anode  loses  by  the  attack  of  the  acid. 

This  would  be  true  if  no  chemical  action  occurred  except  that  directly 
caused  by  the  current.  It  is  a  fact,  however,  that  the  character  of  a 
deposited  metal  often  varies  with  the  strength  of  the  current  by  means 
of  which  it  is  deposited,  or  with  the  strength  of  the  solution  used  as  the 
electrolyte.  Copper  is  sometimes  deposited  in  the  form  of  a  black 
powder  instead  of  a  smooth,  bright  layer  of  metal.  Silver  is  often 
deposited  in  crystals  which  build  across  the  electrolyte  between  the 
electrodes.  Tin  forms  a  "  tree  "  of  tin  crystals,  when  deposited  from 
a  tin  chloride  solution,  the  branches  of  which  spread  out  from  the 
electrode  through  the  solution.  The  greatest  care  must,  therefore,  be 
used  to  get  satisfactory  measurements.  The  direction 'of  the  current 
should  be  determined  by  a  compass  needle  before  the  voltameter  is 
placed  in  the  circuit. 

The  loss  of  the  anode  is  seldom  as  reliable  a  measure  of  the  current 
as  the  gain  of  the  cathode,  because  bits  of  metal  are  liable  to  be 
loosened  from  the  former  and  fall  off,  and  the  anode  also  often  suffers 
from  oxidation. 

158.  Silver  Voltameter.  —  When  a  Silver  Voltameter  is  used  for  the 
measurement   of  a  current,  as   is   assumed   in   the   definition   of  the 
ampere,2  the  electrolyte  is  a  solution  of  the  Nitrate  of  Silver  of  fixed 

1  Articles  62  and  65.  2  Article  96. 


1 66  ELECTRICITY  AND   MAGNETISM 

strength.  The  cathode  is  usually  a  platinum  bowl  upon  which  the  silver 
is  deposited,  and  the  anode  is  a  wire  or  plate  of  pure  silver,  which  is 
wrapped  in  filter  paper  to  keep  bits  of  silver  from  dropping  on  to  the 
cathode.  Before  a  measurement  of  current  is  to  be  made,  the  cathode 
is  very  accurately  weighed,  the  solution  is  then  poured  into  it  and  the 
anode  is  put  in  place.  The  current  is  turned  on  and  continued  for  a 
desirable  number  of  seconds.  It  is  then  stopped,  the  cathode  is 
carefully  washed  and  dried,  and  finally  again  weighed  with  great  care. 
From  its  gain  in  weight,  the  time  the  current  flowed,  and  the  electro- 
chemical equivalent  of  silver,  the  value  of  the  current  is  determined. 

159.  Copper  and  Zinc  Voltameters. —  On  account  of  the  expense 
of  the  silver  consumed  and  the  care  required  in  using  a  silver  voltam- 
eter, it  is  not  satisfactory  for  measuring  currents  exceeding  about  one 
ampere.  For  larger  currents,  a  voltameter  having  copper  plates,  and  a 
solution  of  copper  sulphate  for  electrolyte,  is  generally  used.  A  good 
deposit  is  usually  obtained  if  the  copper  solution  has  a  density  between 
i.io  and  1. 1 8,  as  measured  by  a  hydrometer.  The  action  of  the 
solution  is  improved  by  the  addition  of  a  small  amount  of  sulphuric 
acid. 

The  meter  formerly  used  almost  exclusively  by  Edison  electric  lighting 
companies  to  determine  the  quantity  of  electricity  delivered  per  month 
to  customers,  consists  of  a  shunted  voltameter  with  amalgamated  zinc 
plates  and  an  electrolyte  of  zinc  sulphate. 

The  weight  in  grammes  of  different  metals  deposited  by  one  ampere 
in  one  second  (that  is,  the  electrochemical  equivalents)  is  given  in  a 
table  contained  in  Chapter  V.  The  following  data,  taken  from  that 
table,  give  the  grammes  of  the  gas  or  metal  which  may  be  deposited  for 
each  coulomb  passed  through  either  of  the  voltameters  just  described: — 

METAL  ELECTROCHEMICAL  EQUIVALENT 

Hydrogen,  ......         .0000104 

Silver,         ......         .001118 

Copper,' 000329 

Zinc,  ......         .000338 

One  gramme  (metric  measure)  is  equal  to  15.432  grains,  or  approxi- 
mately thirty-five  one-thousandths  of  an  ounce. 


GALVANOMETERS   AND   VOLTAMETERS  1  67 


PROBLEMS 

fc^' 

A.  How  much  weight  in  grammes  would  the  cathode  of  a  silver  voltameter  gain 

if  a  current  of  exactly  I  ampere  flowed  through  it  for  I  hour?     Ans.  4.025  (approx.). 

B.  A  current  flowing  through  a  copper  voltameter  for  100  minutes  increased  the 
weight  of  the  cathode  10  grammes.     What  was  the  average  current  flowing?    Ans. 
5.07  amperes  (approx.). 

C.  An  Edison  zinc  voltameter,  after  being  on  a  house  -lighting  circuit  for  a  month, 
was  found  to  have  deposited  15.43  grains  of  zinc  upon  the  cathode.     How  many 
ampere  hours  (an  ampere  hour  is  an  ampere  flowing  for  an  hour)  had  been  con- 
sumed during  the  month  in  lighting  the  house  if  ^^th  of  the  total  current  passed 
through  the  meter?     Ans.  80  1  (approx.). 


QUESTIONS 

1.  What  is  a  galvanometer? 

2.  What  is  a  sensitive  galvanometer? 

3.  Why  are  a  large  number  of  turns  used  in  galvanometers  for  measuring  small 
currents,  and  a  small  number  of  turns  in  galvanometers  for  large  currents? 

4.  WThy  is  it  desirable  to  have  the  resistances  of  galvanometers  for  measuring  large 
currents  very  low? 

5.  Why  does  a  galvanometer  coil  act  with  the  greatest  force  upon  its  needle  when 
the  coil  stands  in  a  north  and  south  plane? 

6.  How  can  the  direction  of  a  current  be  found  by  a  galvanometer?    Its  strength? 

7.  What  is  a  tangent  galvanometer? 

8.  What  is  a  sine  galvanometer? 

9.  What  is  a  reflecting  galvanometer? 

10.  Explain  the  use  of  the   lamp,  telescope,  and   scale   in   a   reflecting  galva- 
nometer. 

11.  How  are  galvanometer  needles  suspended  or  supported? 

12.  What  is  a  "  directive  magnet  "?     How  is  it  used? 

13.  What  are  astatic  needles? 

14.  Why  are  astatic  needles  used  in  galvanometers? 

15.  How  are  galvanometer  needles  made? 
1  6.   What  is  a  D'Arsonval  galvanometer? 

17.  How  are  D'Arsonval  galvanometer  coils  supported? 

1  8.  Give  two  advantages  in  favor  of  D'Arsonval  galvanometers. 

19.  What  is  a  "dead-beat"  galvanometer? 

20.  How  are  galvanometers  made  dead-beat? 

21.  What  is  a  galvanometer  constant? 

22.  What  is  a  galvanometer  calibration  curve? 

23.  How  can  a  galvanometer  calibration  curve  be  obtained? 


1 68  ELECTRICITY  AND    MAGNETISM 

24.  If  the  deflections  of  a  galvanometer  were  directly  proportional  to  the  currents, 
how  could  the  relative  strengths  of  two  currents  be  found? 

25.  What  is  a  galvanometer  shunt? 

26.  If  it  is  desired  that  one-thousandth  part  of  a  current  to  be  measured  shall 
flow  through  a  galvanometer,  what  ratio  must  the  resistance  of  the  shunt  bear  to 
that  of  the  galvanometer  ?     Why? 

27.  How  can  a  sensitive  galvanometer  be  used  for  measuring  large  currents? 

28.  What  is  a  voltameter? 

29.  How  can  a  current  be  measured  by  means  of  a  water  voltameter? 

30.  Describe  the  action  in  a  metal  voltameter. 

31.  What  is  likely  to  happen  in  copper,  silver,  and  tin  voltameters  if  the  current  is 
too  strong,  or  the  strength  of  the  solution  is  not  right? 

32.  How  can  a  current  be  measured  by  a  metal  voltameter? 

33.  In  measuring  the  current  by  a  metal  voltameter,  should  the  change  in  the 
weight  of  the  anode  or  of  the  cathode  be  used?     WThy? 

34.  What  is  a  silver  voltameter?     How  is  it  used? 

35.  What  electrolyte  is  used  in  a  silver  voltameter  for  determining  the  inter- 
national ampere? 

36.  Tell  how  you  would  measure  currents  by  copper  and  zinc  voltameters. 

37.  What  is  the  numerical  value  of  the  electrochemical  equivalent  of  silver? 


CHAPTER  XII 


MEASUREMENT  OF  ELECTRICAL   RESISTANCE 

160.  Measurement  of  Resistance  by  Substitution.  —  All  useful  methods 
of  measuring  electrical  resistance  depend  directly  upon  the  indications  of 
Ohm's  Law.     The  simplest  method  of  measuring  a  resistance  is  by  what 
is  called  Substitution.     The  resistance  to  be  measured  is  connected  in 
series  with  a  galvanometer  and  a  constant  battery,1  and  the  deflection 
of  the  galvanometer  is  noted.    Then  the  unknown  resistance  is  removed 
from  the  circuit,  and  a  Variable  Resistance  Box  or  Rheostat  (Fig.  87)  is 
substituted    for    it. 

The  resistance  of 
the  resistance  box 
is  then  adjusted  un- 
til the  galvanome- 
ter deflection  is  the  -=^ 
same  as  before. 

The  resistance  in- 
serted in  the  circuit 
by  means  of  the  box  is  equal  to  the  unknown  resistance,  because  the  gal- 
vanometer shows  that  the  same  amount  of  current  flows  through  the 
circuit  in  the  two  cases,  and  the  total  electrical  pressure  acting  in  the 
circuit  is  the  same  in  each  case,  and  consequently,  according  to  Ohm's 
Law,  the  resistance  of  the  total  circuit  must  be  the  same  in  the  two 
cases.  It  is  necessary  that  no  changes  be  made  in  the  circuit  during 
the  process  besides  the  substitution  of  the  variable  known  resistance  for 
the  unknown  one. 

161.  Resistance  Boxes.  —  Resistance  boxes  of  the  character  referred 
to  in  the  last  article  are  generally  boxes  containing  spools  of  silk-covered 


FiG.  87.  —  Variable  Resistance  Box. 


1  Article  42. 
169 


ELECTRICITY   AND   MAGNETISM 


wire,  each  of  known  resistance,  which  may  be  used  in  electrical  measure- 
ments. German  silver  or  some  similar  alloy,  having  a  comparatively  low 
conductivity  and  a  small  temperature  coefficient,1  is  generally  used  in 
making  the  spools  or  coils  for  resistance  boxes.  In  making  the  coils, 

the  proper  length 
of  wire  for  each  is 
taken  and  doubled 
at  the  middle,  and 
it  is  then  wound 
double  upon  a 
spool.  The  object 
of  doubling  the 
wire  is  to  avoid  the 
effects  due  to  self- 
inductance,  which 
will  be  explained 
later.  After  the 
spools  are  wound, 
they  are  dipped  in 


FIG. 


3. —  Spools  of  Wire  fastened  to  Under  Side  of  Cover  of 
Resistance  Box. 

paraffine,  and  then 

placed  inside  the  box,  and  fastened  to  the  under  side  of  the  top  of 
the  box  by  brass  bolts'  (Fig.  88),  which  also  fasten  a  series  of  brass 
blocks  to  the  upper  side  of  the  top,  as  is 
illustrated  in  Figure  87.  The  individual  ends 
of  each  coil  are  connected  to  adjoining  brass 
blocks,  so  that  all  the  coils  are  in  series  when 
the  blocks  are  not  directly  connected  to- 
gether. This  is  shown  in  Figure  89,  where 
the  ends  a  and  b  of  one  of  the  resistance 
coils  are  fastened  to  the  brass  blocks  L  and 
M,  while  the  ends  c  and  d  of  the  next  coil 
are  fastened  to  the  blocks  M  and  N.  The 

brass  blocks  are  so  arranged  that  they  may  be  connected  together  by 
plugs  which  fit  in  tapering  holes,  as  shown  in  the  figure,  thus  "  short- 
circuiting"  the  coils. 

1  Articles  91  and  98, 


FIG.  89.  —  Arrangement  of 
Brass  Blocks  and  Plugs 
for  Resistance  Box. 


MEASUREMENT  OF   ELECTRICAL  RESISTANCE  171 

If  such  a  resistance  box  is  connected  in  a  circuit  when  all  the  plugs 
are  removed,  the  current  flows  through  all  the  resistance  coils  in  series. 
If  one  of  the  plugs  is  inserted  in  a  hole,  the  corresponding  resistance  coil 
is  Short-circuited,  that  is,  a  negligibly  small  resistance  (that  of  the  plug) 
is  connected  in  parallel  with  it,  and  no  appreciable  current  flows  through 
the  coil.  Since  the  resistance  of  the  plug  is  practically  negligible,  the 
resistance  of  the  circuit  is  reduced,  when  the  plug  is  inserted,  by  the 
amount  of  the  resistance  of  the  corresponding  coil.  The  resistance  of  a 
box  may,  therefore,  be  varied  at  will  by  simply  inserting  or  removing 
plugs. 

Resistance  boxes  generally  have  a  series  of  coils  of  different  resist- 
ances, usually  given  in  tenths,  units,  tens,  hundreds,  etc.,  of  ohms. 
The  final  adjustment  by  the  manufacturer  of  the  coils  of  a  fine  resist- 
ance box,  so  that  they  may  all  have  the  right  resistance,  is  a  matter 
requiring  great  care.  It  is  effected  by  soldering  more  or  less  of  the 
doubled  ends  of  the  wire  together  after  the  spool  is  mounted  in  its  box. 
In  order  that  the  adjustment  may  be  made  in  this  way  it  is  necessary  that 
the  resistance  of  the  coil  when  wound  on  the  spool  shall  be  a  little  greater 
than  the  desired  final  value.  Great  care  must  be  taken  when  adjusting 
coils  to  avoid  errors  due  to  the  temperature  of  the  coil  changing,  since 
the  wires  are  likely  to  become  heated  by  the  soldering. 

162.  Wheatstone  Bridge.  —  The  measurements  for  determining  the 
exact  value  of  the  coils  are  made  by  what  is  called  a  Wheatstone  Bridge, 
after  Wheatstone,  an  English  scientist  and  inventor.     This  consists  of  an 
arrangement  of  resistance  coils  which  are  used  in  a  combination  with  a 
battery  and  galvanometer,  as  shown  diagram matically  in  Figure  90.    In  the 
figure,  A,  B,  and  R  represent  resistance  boxes  with  coils  of  known  re- 
sistance ;  X  is  the  resistance  to  be  measured ;  F  and  G  are  a  battery 
and  a  galvanometer  ;  K±  and  K%  are  Keys  placed  in  the  circuits  with  the 
battery  and  galvanometer,  by  means  of  which  the  circuits  may  be  made 
and  broken ;  M,  N,  O,  and  P  are  points  where  the  various  Bridge  Cir- 
cuits are  connected  together. 

163.  Measurement  of  Resistance  by  Wheatstone  Bridge.  —  From  an 
application  of  the  law  of  the  fall  of  potential  along  a  resistance  as  deduced 
from  Ohm's  Law,1  it  is  easy  to  see  how  the  resistance  of  the  coil  X  is 

1  Article  106. 


1/2 


ELECTRICITY  AND   MAGNETISM 


determined  by  this  device.  Suppose  the  Battery  Key  K±  (Fig.  90)  is 
depressed,  then  the  current  flows  from  the  battery  through  the  key  to 
the  point  P.  Here  it  divides,  and  part  goes  to  O  by  way  of  Mt  and  the 
other  part  by  way  of  N.  From  O  the  current  returns  to  the  battery. 
The  points  P  and  O  are  at  a  certain  difference  of  electrical  pressure 
(which  we  call  E)  that  depends  upon  the  battery,  and  the  fall  of  press- 
ure from  />  to  O  by  way  of  either  M  or  N  is  equal  to  E.  The  fall  of 
pressure  between  P  and  M  is  (according  to  the  law  that  the  fall  of  press- 

OC 

ure  is  proportional  to  the  resistance  passed  over)  E—  — -  where  x  and 


A0 


FIG.  90.  —  Diagrammatic  Illustration  of  Wheatstone  Bridge  and  its  Connections. 

b  represent  the  resistance  of  the  branches  of  the  bridge  X  and  B  re- 
spectively.    In  the  same  way  the  fall  of  pressure  between  P  and  N  is 

equal  to  E  —      -  where  r  and  a  represent  the  resistances  of  R  and  A. 

T  -p  dt 

If  the  fall  of  pressure  between  /'and  M  is  greater  than  that  between 
P  and  N  the  point  M  is  at  a  lower  pressure  than  N,  and  if  the  Galva- 
nometer Key  K.2  is  depressed,  a  current  will  flow  from  N  to  M  through 
the  galvanometer,  deflecting  the  needle.  Now,  if  the  resistance  r  is 
increased  until  the  fall  of  pressure  between  P  and  N  is  the  greater,  a 
current  will  flow  from  M  to  N  when  the  galvanometer  key  is  depressed, 
and  the  needle  will  be  deflected  in  the  opposite  direction. 


MEASUREMENT  OF   ELECTRICAL   RESISTANCE  173 

Finally,  if  the  resistance  of  r  is  so  adjusted  by  arranging  the  plugs 
that  the  fall  of  pressure  between  P  and  M  is  equal  to  the  fall  of  pressure 
between  P  and  N,  the  pressures  at  the  points  M  and  N  are  equal,  and 
no  current  will  flow  through  the  galvanometer  when  the  key  is  depressed, 
and  the  needle  will  not  be  deflected.  The  bridge  is  then  Balanced.  In 
this  case  „ 


x  4-  b          r  -\-  a 

^C          V 

or,  what  is  the  same  thing,  -  =  — 

b      a 

From  this  proportion  we  get  x  =  r--    That  is,  the  unknown  resistance 

a 

of  X  is  equal  to  the  resistance  of  R,  multiplied  by  the  resistance  of  B, 
divided  by  that  of  A.     Put  in  the  form  of  a  proportion  this  may  be  written 

x  is  to  r  as  b  is  to  a  ;  or  x  is  to  b  as  r  is  to  a. 

The  solution  of  either  of  these  proportions  gives  the  results  presented 
above.  If  a  and  b  are  equal,  and  the  bridge  is  balanced,  r  must  be  equal 
to  x,  so  that  the  resistance  of  the  unknown  Branch  or  Arm  of  the  bridge 
is  given  at  once  by  the  resistance  of  the  coils  in  circuit  at  R.  The  resis- 
tance of  A,  in  Fig.  90,  is  ten  times  as  great  as  that  of  B,  and  therefore 
the  resistance  of  R  is  ten  times  that  of  X,  and  x  is  therefore  equal  to 
1.5  ohms.  If  the  resistance  of  £  had  been  ten  times  as  great  as  that  of 
A  when  the  bridge  was  balanced,  R  would  have  been  one-tenth  as  great 
as  X  instead  of  ten  times  as  great.  The  arms  A  and  B  are  generally 
called  the  Ratio  Anns  of  the  bridge,  and  R  the  rheostat.  In  measuring 
a  resistance  it  is  well  to  begin  with  the  ratio  arms  of  equal  resistance, 
changing  their  ratio  to  yL,  -j-J-g-,  etc.,  as  is  shown  to  be  desirable  as  the 
balancing  proceeds.  If  it  is  found  that  the  unknown  resistance  is  of 
low  value,  the  bridge  arms  should  be  of  as  low  resistance  as  will  give  the 
needed  ratio,  while  for  high  resistance  the  opposite  is  true. 

PROBLEMS 

A.  Suppose  a  resistance  is  to  be  measured  by  a  bridge,  and  after  the  bridge  is  bal- 
anced the  rheostat  resistance  (^?)  reads  15.6  ohms,  while  the  ratio  arms  are   100  (A~) 
to  10  {B},  what  is  the  value  of  the  unknown  resistance?     Ans.    1.56  ohms. 

B.  Suppose  R  reads  2600,  and  A  and  B  are   10  and  1000  respectively  when  a 
balance  is  obtained,  what  is  X?    Ans.    260,000  ohms. 


174 


ELECTRICITY  AND   MAGNETISM 


C.  If  the  R  arm  of  a  bridge  contains  resistance  coils  from  I  to  10,000  ohms,  what 
will  you  make  the  ratio  of  the  ratio  arms  in  order  to  measure   1,000,000  ohms, 
supposing  that  these  arms  each  contain  coils  of  I,  10,  100,  and  1000  ohms  resistance? 
Ans.   A :  B : :  1:  IOOO. 

D.  If  in  Example  C  the  unknown  resistance  is  .01  ohms,  what  must  be  the  ratio  ? 
Ans.    A:  B::  IOOO:  i. 

E.  If  the  R  arm  of  a  bridge  contains  resistance  coils  from  I  to  10,000  ohms,  what 
resistance  would  you  give  the  three  known  arms  to  measure  625,500  ohms  ?     Ans. 
R  =  6255,  A  =  10,  and  B  —  1000. 

F.  If  in  Example^  the  unknown  resistance  was  .625,  what  would  be  the  arms  ? 
Ans.    R  =  625,  A  =  1000,  and  B  =  I. 

164.  Usual  Form  of  Wheatstone  Bridge.  —  The  Wheatstone  bridges 
that  are  commonly  used  are  not  made  up  from  three  separate  resistance 
boxes  as  indicated  in  Figure  90.  The  common  forms  of  Wheatstone 
bridge  contain  all  the  resistance  coils  in  one  box,  and  the  coils  are  con- 
nected in  such  a  way  that  they  form  a  bridge.  Binding  posts  (gener- 
ally marked  B,  G, 
and  X,  or  R)  are 
arranged  at  the 
proper  points  for 
connecting  the  bat- 
tery, galvanometer, 
and  unknown  resist- 
ance which  is  to 
be  measured  to  the 
bridge. 

Figure  9 1  shows 
such  a  bridge  made 
up  in  a  box  so  as 
to  be  portable.  At 
the  front  are  seen 
1  the  battery  and 


FIG.  91.  — Bridge  of  Posi-office  Pattern. 


galvanometer    keys. 

This  form  of  bridge 
is  often  called  the  Post-office  Pattern,  because  its  arrangement  is 
similar  to  the  bridge  used  by  the  British  department  of  postal  tele- 
graphs. 


MEASUREMENT   OF   ELECTRICAL   RESISTANCE  175 

Figure  92  shows  another  way  in  which  the  resistance  coils  are  often 
arranged  to  make  a  very  accurate  and  convenient  bridge  for  use  in 
laboratories  where  it  may  be  permanently  fixed. 


FIG.  92.  —  Bridge  of  Dial  Pattern. 

165.  Accuracy  of  Bridge.  —  Measurements  of  resistance  may  be  made 
with  a  fine  bridge  to  a  remarkable  degree  of  accuracy.    In  fact,  the  ease 
and  accuracy  to  be  obtained  in  bridge  measurements  are  only  rivalled  in 
weighing  with  fine  balances.     It  is  not  unusual  to  have  the  resistance 
coils  of  a  fine  bridge  adjusted  so  accurately  that  the  error,  when  used  at 
some  fixed  temperature,  is  less  than  -fa  of  one  per  cent  of  their  desired 
value  which  is  represented  by  a  standard  coil, —  that  is,  their  error  is  less 
than  two  parts  out  of  ten  thousand  when  the  coils  are  at  the  fixed  tem- 
perature.     In  adjusting  the  coils  of  a  resistance  box  so  closely,  or  in 
accurately  measuring  a  resistance  by  a  bridge,  careful  account  must  be 
taken  of  the  temperature.     Even  if  the  resistance  coils  of  a  bridge  were 
exactly  correct  at  one  temperature  they  would  not  be  correct  at  another ; 
but  corrections  may  be  made  for  the  Temperature  Error. 

166.  Testing  Sets.  —  It  is  frequently  convenient  to  have  a  portable 
bridge  which  is  entirely  self-contained  —  that  is,  the  box  of  which  con- 
tains the  galvanometer  and  battery  as  well  as  the  resistance  coils  and 
keys.     In  this  case  all  that  is  necessary  to  make   a   measurement   of 
resistance  is  to  connect  the  unknown  resistance  to  the  proper  binding 
posts,  press  the  keys,  and  adjust  the  plugs  till  the  galvanometer  gives  no 
deflection.    Such  bridges  are  called  Testing  Sets.     One  is  shown  in  Fig- 
ure 93.     The  resistance  coils  of  testing  sets  and  other  less  expensive 


ELECTRICITY  AND  MAGNETISM 


bridges  are  usually  adjusted  with  such  care  that  their  error  at  ordinary 
temperatures  does  not  exceed  ^  of  one  per  cent. 

167.  Manipulation.  —  When 
resistance  measurements  are  be- 
ing made  with  a  bridge,  the  bat- 
tery key  should   be   depressed 
before  the  galvanometer  key,  or 
irregular  and  incorrect  indica- 
tions will  often  be  given  on  ac- 
count of  the  self-induction  of 
the  unknown  resistance.     This 
is    particularly   true   when    the 
unknown  resistance  consists  of 
the  windings  of  an  electromag- 
net or  any  of  the  windings  of 
a  dynamo.     Great  care  should 
always  be  exercised  not  to  in- 
jure the    galvanometer   or   the 
fine  wire  coils  by  passing  too 
great  a  current  through  them. 

168.  Meter  Bridges;   Meas- 
urement of  Low  Resistances.  — 

For  very  accurate  comparisons  of  two  resistances,  as  when  the  value  of 
a  standard  resistance  coil  is  to  be  determined  in  terms  of  a  mercury 
column  or  of  another  coil,  the  Wheatstone  bridge  is  made  up  in  another 
form  (Fig.  94).  Here  we  have  two  arms  of  the  bridge,  A  and  B,  made 


FIG.  93.  — Commercial  Testing  Set. 


FlG.  94.  —  Meter  or  Slide  Wire  Bridge. 

up  of  a  uniform  wire  of  high  resistance  and  small  temperature  coefficient 
The  other  two  arms  contain  the  two  coils.  The  galvanometer  terminal 
corresponding  to  M  (Fig.  90)  is  made  up  by  means  of  a  binding  post, 
but  the  terminal  corresponding  to  N  is  arranged  so  that  contact  may  be 


MEASUREMENT  OF  ELECTRICAL   RESISTANCE  177 

made  at  any  point  along  the  Bridge  Wire.  When  the  galvanometer  con- 
tact is  placed  at  the  point  on  the  bridge  wire  which  gives  a  balance,  the 
resistances  of  the  parts  of  the  bridge  on  each  side  of  the  galvanometer 
contact  are  in  the  same  ratio  as  the  two  resistance  coils,  according  to  the 
bridge  formula  already  developed. 

When  the  bridge  wire  is  calibrated,  that  is,  when  the  resistance  per 
centimeter  of  length  at  every  point  of  the  wire  is  determined,  the  ratio 
of  the  resistance  of  the  two  coils  is  given  by  the  ratio  of  the  resistances 
of  the  two  parts  of  the  wire.  When  the  bridge  wire  is  very  uniform  and 
the  measurement  is  not  required  to  be  very  exact,  the  resistances  of  the 
two  parts  of  the  wire  may  be  taken  to  be  proportional  to  their  lengths. 
Bridge  wires  may  be  made  of  German  silver,  but  those  intended  for 
very  accurate  measurements  are  usually  made  of  an  alloy  containing 
platinum  and  silver,  or  platinum  and  iridium.  Bridges  of  this  form  are 
usually  called  Slide  Wire,  Divided  Wire,  or  Meter  Bridges. 


PROBLEMS 

A.  The  lengths  of  the  bridge  wire  on  either  side  of  the  galvanometer  terminal 
of  a  slide  wire  bridge  are  each  50  cm.     If  the  R  arm  contains  1.2  ohms  when  a 
balance  is  obtained,  what  is  the  unknown  resistance?     Ans.  1.2  ohms. 

B.  Suppose  in  Example  A  the  length  of  the  wire  on  the  side  next  the  unknown  re- 
sistance had  been  only  4  centimeters,  what  would  have  been  the  unknown  resistance? 
Ans.  .05  ohms. 

169.  Measurements  of  High  Resistances.  —  Measurements  of  very 
great  resistances,  such  as  the  Insulation  Resistance  of  a  well-insulated 
wire  between  its  conductor  and  the  ground,  often  require  a  higher 
power. than  may  be  conveniently  reached  by  a  bridge.  In  this  case,  a 
fine  reflecting  galvanometer  and  a  large  testing  battery  are  generally  used. 
The  Testing  Battery  usually  consists  of  silver  chloride  cells  put  up 
in  sets  of  50  or  100  cells  in  a  box,  so  as  to  be  portable.  The 
galvanometer  and  battery  are  connected  in  series  with  some  large 
known  resistance,  and  the  deflection  of  the  galvanometer  is  read. 
Then  the  known  resistance  is  removed  from  the  circuit  and  that 
which  it  is  desired  to  measure  is  inserted  in  its  place.  The  deflection 
of  the  galvanometer  is  again  read,  and  the  unknown  resistance  may  be 

N 


1 78  ELECTRICITY   AND   MAGNETISM 

calculated  from  the  ratio  of  the  two  deflections  and  the  value  of  the  known 
resistance.  This  is  a  modification  of  the  substitution  method  described 
at  the  opening  of  this  chapter.  The  known  or  Standard  Resistance  is 
usually  from  25,000  to  1,000,000  ohms  in  resistance.  One  million  ohms 
is  called  a  Megohm,  the  prefix  "  meg  "  coming  from  a  Greek  word  mean- 
ing great. 

170.  Use  of  Shunt  Boxes.  —  The  insulation  resistances  of  wires 
and  cables  that  are  measured  by  this  process  are  frequently  as  great 
as  thousands  of  megohms,  so  that  it  is  necessary  to  use  a  very  fine 
galvanometer  to  get  readable  deflections  through  them,  and  the  gal- 
vanometer must  be  shunted1  when  a  deflection  is  taken  with  the 
standard  resistance  in  circuit.  Galvanometers  usually  have  correspond- 
ing shunt  boxes  sold  with  them  which  have  three  coils  marked  respec- 
tively ^-,  -gig-,  ^-9-9.  When  the  shunt  box  is  connected  in  parallel  with  the 
galvanometer,  either  of  these  shunts  may  be  placed  in  the  circuit  by 
means  of  a  plug,  or  the  shunt  circuit  may  be  broken.  When  a  shunt 
is  plugged  into  circuit,  y1^,  y^-,  or  y^-g-  part  of  the  whole  current 
flows  respectively  through  the  galvanometer. 

As  an  example,  suppose  that  it  is  desired  to  measure  the  insulation  re- 
sistance of  an  electric  light  cable  one-half  mile  long ;  a  fine  galvanome- 
ter, a  testing  battery  of  200  cells,  and  a  standard  resistance  of  one-half 
a  megohm  being  available.  When  connected  up  and  shunted  by  the  9-9-9- 
shunt,  the  galvanometer  gives  a  deflection  of  one  hundred  scale  divi- 
sions. Then  its  constant,  or  the  resistance  of  the  circuit  in  megohms 
which  would  be  indicated  by  a  deflection  of  one  scale  division  when  the 
galvanometer  is  not  shunted,  is  100  x  1000  x  \  =  50,000,  since  1000  is 
the  multiplying  power  of  the  shunt  and  %  the  value  of  the  standard 
resistance  in  megohms.  Now  when  the  standard  resistance  is  re- 
moved from  the  circuit  and  in  its  place  one  end  of  the  circuit  wire 
is  attached  to  the  conductor  of  the  cable  and  the  other  end  to  the 
ground,  the  reading  of  the  galvanometer  without  a  shunt  is,  we  will  say 
for  illustration,  50.  The  insulation  resistance  of  the  cable  is  -^T^  =  1000 
megohms. 

The  insulation  resistance  of  a  similar  cable  for  a  length  of  one  mile 
is  500  megohms,  since  the  cable  which  was  measured  is  one-half  mile 

1  Article  154. 


MEASUREMENT  OF   ELECTRICAL   RESISTANCE  179 

long  and  the  paths  for  the  current  to  leak  out  of  the  two  half-miles 
which  constitute  one  mile  are  in  parallel. 

Other  methods  of  measuring  high  resistances  and  special  methods  of 
measuring  very  low  resistances  are  sometimes  used,  but  they  need  not 
receive  attention  here. 

PROBLEMS 

A.  Suppose  a  galvanometer  of  negligible  resistance  without  a  shunt  gives  a  deflec- 
tion  of  100  when  connected  in  series  with  a  battery  and  standard  resistance  of 
100,000  ohms.     If  the  deflection  becomes  25  when  an  unknown  resistance  is  sub- 
stituted, what  is  the   galvanometer  constant,  and   what   the    unknown   resistance? 
Ans.  10  megohms  ;   .4  of  a  megohm. 

B.  If  in  place  of  the  standard  resistance  terminals  of   Example  A   one  end  of 
an  insulated  telegraph  line  and  a  connection  to  the  ground  be  substituted,  and  the 
deflection  becomes  50,  what  is  the  insulation  resistance  of  the  line?     Ans.  200,000 
ohms. 

C.  A  galvanometer  when  used  with  a  certain  battery  has  a  constant  of  20,000 
megohms.     If  an  unknown  resistance  be  placed  in  circuit  and  the  deflection  is  100 
when  a  -fa  shunt  is  used,  what  is  the  resistance?     Ans.  2  megohms. 

D.  Suppose  a  reflecting  galvanometer  shunted  with  the  ^¥  shunt  gives  a  deflec- 
tion of  80  when  using  a  certain  battery,  the  standard  resistance  being  5,000  ohms; 
then  suppose  the  deflection  is  120  with  the  \  shunt  when  a  certain  resistance  is  sub- 
stituted for  the  standard,  other  things  being  unchanged,  what  is  the  value  of  the 
resistance?     Ans.  ^megohm. 

E.  If  the  battery  and   galvanometer  of  Example  D   be   connected  to  another 
unknown    resistance    and    the    deflection    becomes    50   when  a   fa   shunt   is   used, 
what  is  the  resistance?     Ans.  80,000  ohms. 

F.  A  galvanometer  of  1000  ohms  resistance  deflects  one  division  when  .oooooi 
of  an  ampere  passes  through  it  ;   what  is  the  resistance  of  a  coil  which  is  placed 
in  series  with  it  and  a  battery  of  2  volts  pressure,  if  the  deflection  then  is  50  scale 
divisions?     Ans.  39,000  ohms. 

171.  Volt  and  Current  Method  of  measuring  Resistance.  —  The  state- 
ments of  Article  92  make  it  clear  that  the  resistance  of  a  wire  may  be 
calculated  from  Ohm's  Law,  provided  that  we  know  the  pressure  across 
the  terminals  of  the  resistance  to  -be  measured  and  the  current  passing 
through  it.  The  formula  is 


SITY 


i8o 


ELECTRICITY   AND   MAGNETISM 


This  is  a  very  satisfactory  and  common  method.  A  voltmeter  is  used 
for  measuring  the  pressure,  and  an  amperemeter  for  measuring  the  cur- 
rent. The  connections  are  as  shown  in  Figure  95.  The  method  and  the 
instruments  will  be  explained  in  a  later  chapter. 


STORAGE  BATTERY 


FlG.  95.  —  Arrangements  for  measuring  Resistance  by  Amperemeter  and  Voltmeter. 


QUESTIONS 

1.  Explain  how  to  measure  resistance  by  substitution. 

2.  Why  does  the  galvanometer  constant  not  need  to  be  known  in  measuring 
resistance  by  substitution? 

3.  What  precaution    must  be    observed  with  reference  to  connecting  wires  in 
measuring  resistance  by  substitution? 

4.  What  is  a  rheostat? 

5.  Describe  the  construction  of  a  resistance  box. 

6.  Why  is  German  silver  wire  used  in  resistance  boxes? 

7.  Why  is  the  wire  of  resistance  boxes  wound  double? 

8.  If  a  plug  is  inserted  into  one  of  the  holes  of  a  resistance  box,  is  resistance 
inserted  in  or  removed  from  the  circuit? 

9.  How  is  the  resistance  of  a  resistance  coil  adjusted?    How  is  the  wire  insulated? 

10.  What  is  a  Wheatstone  bridge? 

11.  How  is  a  Wheatstone  bridge  made? 


MEASUREMENT  OF   ELECTRICAL   RESISTANCE  l8l 

12.  When  is  a  bridge  said  to  be  balanced? 

13.  What  proportion  must  the  resistances  of  the  bridge  arms  have  in  order  that 
the  bridge  shall  be  balanced? 

14.  If  the  galvanometer  needle  deflects  to  the  right  when  the  fall  of  pressure  is 
greater  in  R  than  in  X,  what  will  happen  if  the  fall  of  pressure  in  X  becomes  the 
greater? 

15.  When  a  bridge  is  balanced,  what  is  the  unknown  resistance  x  equal  to  in 
terms  of  the  resistances  of  the  other  arms? 

1 6.  What  are  the  "  ratio  arms  "  of  a  bridge? 

1 7.  When  the  ratio  arms  A  and  B  are  equal  in  a  bridge,  why  must  R  and  X  be 
equal  when  the  bridge  is  balanced? 

1 8.  If  ratio  arm  A  (Fig.  90)  is  ten  times  larger  than  B,  what  will  be  the  relative 
sizes  of  ^  and  X  when  the  bridge  is  balanced?     What  if  B  is  larger  than  A! 

19.  Describe  the  post-office  pattern  bridge. 

20.  What  is  the  temperature  error  in  a  bridge? 

21.  How  accurate  can  bridges  be  made? 

22.  Describe  a  testing  set. 

23.  Why  should  the  battery  key  of  a  bridge  be  depressed  before  the  galvanometer 
key? 

24.  What  is  a  meter  bridge? 

25.  Plow  is  a  meter  bridge  used  ? 

26.  What  is  the  slide  wire  in  a  meter  bridge  made  of  ? 

27.  Why  is  a  meter  bridge  especially  desirable  for  measuring  low  resistances? 

28.  How  may  insulation  resistances  be  measured? 

29.  What  is  the  use  of  a  shunt  in  measuring  very  high  resistances  by  means  of 
a  galvanometer? 

30.  How  can  a  high  resistance  be  measured  by  using  a  galvanometer,  when  its 
constant,  as  explained  in  Article  153,  is  known? 

31.  Should  a  galvanometer  have  a  great  or  a  small  number  of  turns  for  measuring 
high  resistances?     For  measuring  low  resistances?     Why? 

•  32.    How  can  the  resistance  of  a  circuit  be  determined  by  measurements  of  current 
and  pressure? 

33.   When  is  a  coil  or  circuit  "short-circuited"? 


CHAPTER   XIII 

MEASUREMENT  OF  ELECTRIC  CURRENTS   AND   PRESSURES 

172.  Principles  of  Instruments  for  Current  Measurement.  —  We  have 
already  seen  that  electric  currents  may  be  measured  by  taking  advan- 
tage of  three  different  and  independent  effects  of  the  current.  These 

are  :  — 

1 .  The  electrochemical  effect. 

2.  The  magnetic  effect. 

3.  The  heating  effect. 

By  taking  advantage  of  the  first  effect  we  may  measure  currents  by 
the  use  of  voltameters;1  as  a  result  of  the  second  effect  we  measure 
currents  by  means  of  galvanometers ; 2  from  the  third  effect  we  may 
measure  currents  by  means  of  the  expansion  of  a  wire  which  is  heated 
by  the  passage  of  the  current  through  it.3 

Voltameters,  as  said  in  Chapter  XI,  are  principally  used  for  calibrat- 
ing galvanometers  or  for  similar  purposes,  as  they  are  not  sufficiently 
convenient  for  general  use.  The  liquid  must  be  kept  fairly  pure  and  of 
the  proper  density.  Conveniences  must  be  available  for  cleaning,  dry- 
ing, and  accurately  weighing  the  cathodes.  In  order  that  a  satisfactory 
measurement  of  the  current  may  be  made,  the  period  during  which  "it 
flows  through  a  voltameter  must  be  considerable.  They  have  been  found 
particularly  useful  for  only  one  purpose  in  everyday  measurements ; 
that  is,  as  a  meter  such  as  was  formerly  used  by  many  of  the  Edison 
Illuminating  Companies.4  Voltameters  were  used  for  this  purpose  in 
the  early  days  of  electric  lighting  with  incandescent  lamps,  and  have 
continued  in  use  until  lately ;  but  even  for  that  purpose  they  have  been 
displaced  by  the  excellent  mechanical  meters  that  are  now  to  be  had. 

1  Articles  155  to  159.  3  Article  112. 

2  Articles  144  to  154.  4  Article  159. 

182 


MEASUREMENT   OF   CURRENTS   AND  PRESSURES  183 

173.  Amperemeters  and  their  Uses.  —  Nearly  all  our  common  instru- 
ments for  measuring  currents  depend  upon  the  magnetic  effect  of  the 
current  for  their  indications,  and  are  really  modified  galvanometers  with 
pointers  to  show  the  deflections. 

Galvanometers  or  other  instruments  intended  especially  for  convenient 
use  in  everyday  measurements  of  currents  are  generally  called  Ampere- 
meters or  Ammeters,  because  they  measure  amperes.  Amperemeters 
are  made  in  various  forms,  all  more  or  less  portable.  Almost  every 
manufacturer  of  dynamos  or  other  electrical  machinery  manufactures 
amperemeters  which  may  be  used  in  service  with  his  machines.  Ampere- 
meters are  universally  used  where  electricity  is  used,  and  they  are  made 
to  measure  currents  consisting  of  only  a  few  thousandths  of  an  ampere, 
or  Milliamperes  (inilli  comes  from  a  Latin  word  meaning  thousand),  up 
to  the  enormous  currents  generated  by  some  of  the  larger  electric  light- 
ing plants,  reaching  to  thousands  of  amperes.  In  large  electric  lighting 
plants  or  works,  many  amperemeters  may  be  seen  mounted  on  the  wall 
or  on  a  board  among  switches  for  controlling  the  current.  These  are 
used  to  show  the  dynamo  attendants  how  much  current  is  being  gen- 
erated by  the  plant  at  any  moment,  and  what  proportion  is  furnished 
by  each  dynamo. 

Amperemeters  are  used  in  laboratories  to  determine  the  current  used 
in  experiments,  and  to  determine  the  amount  of  current  used  in  the 
operation  of  electric  lamps,  electric  motors,  or  other  electric  devices. 
Physicians  use  amperemeters  to  measure  the  currents  used  in  the  elec- 
trical treatment  of  their  patients.  For  the  latter  purpose  the  currents  are 
usually  measured  in  milliamperes.  The  currents  used  in  telegraphy  are 
also  usually  measured  in  milliamperes,  and  the  currents  used  in  operat- 
ing telephones  are  usually  measured  in  Microamperes,  or  millionths  of 
amperes  (micro  comes  from  a  Greek  word  meaning  small).  Ampere- 
meters that  are  specially  made  to  measure  thousandths  of  amperes,  or 
milliamperes,  are  called  Milliamperemeters.  Externally,  milliampere- 
meters  look  like  ordinary  amperemeters,  to  which  they  bear  the  same 
relation  that  a  very  sensitive  galvanometer  bears  to  a  similar  but  less  sen- 
sitive instrument.  Amperemeters  measure  the  current  flowing  through 
a  circuit,  and  they  are  therefore  connected  in  series  in  the  circuit. 

174.  Mechanism  of  Magnetic  Amperemeters. — The  mechanical  details 


1 84 


ELECTRICITY   AND    MAGNETISM 


entering   into  the  construction  of  magnetic   amperemeters  differ  very 
widely.     They  may  be  roughly  divided  into  three  classes  :  — 

1.  Those  having  soft  iron  parts  which  are  moved  by  the  magnetic 
attraction  set  up  by  the  current  in  the  coils  of  the  instrument. 

2.  Those  having  permanently  magnetized  parts  which  are  acted  upon 
by  the  magnetic  force  set  up  by  a  current  in  the  coils  of  the  instrument, 
either  the  coil  or  the  magnet  moving. 

3.  Those  having  no  iron  in  their  construction,  but  having  two  coils, 
one  of  which  is  moved  by  magnetic  forces  exerted  between  them  when 
a  current  flows  in  both. 

The  moving  parts  of  amperemeters  are  usually  mounted  on  pivots 
which  are  carefully  wrought  to  reduce  the  friction  to  a  small  value. 

If  the  magnetic  force  caused  by  a  current  in  the  coils  of  an  ampere- 
meter had  nothing  except  the  friction  to  overcome,  every  current  would 
pull  the  pointer  entirely  across  the  scale  to  the  stop.  It  is  desirable  to 
construct  the  instrument  so  that  the  movement  of  the  pointer  is  propor- 
tional to  the  current  in  the  windings,  and  a  proper  force  must  therefore 
be  arranged  to  hold  the  pointer  back.  This  may  be  done  by  properly 
Counter- weighting  the  moving  parts,  so  that  the  magnetic  force  must 

raise  them  against  the  force 
of  gravity,  or  by  arranging 
a  proper  spring  to  oppose 
the  magnetic  force.  Figure 
96  shows  an  instrument  in 
which  a  curved  core  G  of 
soft  iron  wire  is  drawn  into 
a  solenoid  of  wires  when 
the  current  flows  through 
the  winding.  The  weight 
of  the  moving  parts  of  the 
instrument  serves  to  keep 
the  pointer  at  zero  when  no 
FIG.  96.— Simple  Form  of  Amperemeter.  current  flows.  When  a  cur- 

rent flows,  it  exerts  an  at- 
traction on  the  iron  wire  core,  which  overcomes  the  effect  of  the  weight 
of  the  moving  parts,  the  iron  core  is  attracted  into^  the  coil  a  certain 


MEASUREMENT  OF  CURRENTS  AND   PRESSURES 


185 


distance,  as  illustrated  in  the  Figure,  and  the  pointer  moves  proportion- 
ally. This  instrument  evidently  belongs  to  the  first  class. 

Instruments  of  the  first  class  may  be  cheaply  constructed,  and  for- 
merly were  commonly  made  by  dynamo  builders  for  use  in  electric  light 
plants. 

175.  Amperemeters  using  Permanent  Magnets.  —  Instruments  hav- 
ing soft  iron  in  their  moving  parts  cannot  readily  be  made  extremely 


FIG.  97.  —  Plan  of  Weston  Amperemeter. 

accurate  because  the  iron  does  not  always  respond  equally  to  the  same 
magnetic  changes  on  account  of  its  coercive  force  ; l  consequently  instru- 
ments of  the  first  class  can  only  be  used,  as  a  rule,  where  great  accuracy 
is  not  essential.  It  is  sufficient  for  the  amperemeters  used  in  many 
electric  plants  to  be  correct  within  five  per  cent,  and  instruments  of  the 
first  class  serve  very  well.  For  testing  which  requires  greater  accuracy, 
instruments  which  belong  to  the  second  and  third  classes  must  be  used. 

1  Article  127. 


1 86 


ELECTRICITY  AND   MAGNETISM 


These  can  be  made  so  that  their  readings  do  not  vary  more  than  one- 
half  of  one  per  cent  from  true  values  when  they  are  used  with  proper 
care. 

Figure  97  shows  a  Weston  amperemeter,  which  is  practically  a 
D'Arsonval  galvanometer  with  the  moving  coil  mounted  on  pivots  and 
arranged  with  a  pointer  to  play  over  a  scale  ;  and  the  whole  is  arranged 
in  a  very  convenient,  portable  form  in  a  self-contained  case  which  is  not 
shown  in  the  figure.  This  instrument  is  a  representative  of  the  second 
class.  AA,  in  the  figure,  are  the  Binding  Posts  of  the  instrument  by 
means  of  which  wires  may  be  connected  to  the  instrument.  Large  wires 
WW  run  from  these  to  the  shunt  E,  and  small  wires  ww  to  the 
armature  coil  C.  This  coil  is  mounted  on  pivots  so  that  it  may  move 
between  the  pole  pieces  of  the  permanent  magnet  MMM.  When  a 
current  flows  through  the  coil  it  tends  to  turn  so  that  its  magnetism 
may  be  parallel  to  the  lines  of  force  of  the  permanent  magnet.  The 
motion  is  opposed  by  the  springs  DD,  so  that  it  is  proportional  to  the 
current.  The  pointer  B  is  attached  to  the  coil  and  moves  over 

the  scale  S,  so  as  to  indicate  the  amount 
of  current  flowing  through  the  instrument. 
The  object  of  the  shunt  E  is  explained  in 
Article  181.  Figure  98  shows  an  end  view 
of  one  of  these  instruments  with  a  portion 
of  the  construction  cut  away  so  as  to  show 
the  works.  The  movable  coil  and  per- 
manent magnet  are  clearly  visible  in  both 
of  the  illustrations.  Similar  letters  in 
Figures  97  and  98  refer  to  the  same 
parts.  The  soft  iron  cylinder,  which  is 
so  plainly  shown  in  Figure  98,  is  used 
to  improve  the  magnetic  circuit  of  the 

magnet.  It  is  stationary,  and  the  conductors  of  the  coil  move  between 
it  and  the  pole  pieces  of  the  magnet.  A  cyclindrical  soft  iron  core 
of  the  same  character  is  shown  in  the  D'Arsonval  galvanometer  of 
Figure  83. 

Weston  or  similar  amperemeters  are  used  a  great  deal  where  accurate 
portable  current  measuring  instruments  are  required,  because  they  are 


FIG.  98.  —  Sectional  End  View  of 
Weston  Amperemeter. 


MEASUREMENT  OF  CURRENTS  AND   PRESSURES 


I87 


accurate,  convenient,  and  well  made.  During  the  past  few  years  the 
best  generating  stations  have  discarded  the  inaccurate  forms  of  ampere- 
meters previously  described,  and  have  adbpted  superior  instruments 
belonging  to  this  class. 

176.  Electrodynamometers.  —  Magnetic  instruments  belonging  to  the 
third  class  are  really  not  galvanometers,  but  are  called  Electrodynamom- 
eters, because  their  indications  are  caused  by  the  magnetic  pull  of  the 
fixed  and  movable  coils  upon  each  other  which  is  caused  by  the  current 
flowing  in  them.  Figure  99  shows  the  ordinary  form  of  the  electro- 
dynamometer  when  arranged  for 
use  as  an  amperemeter.  This 
is  often  called  the  Siemens  Elec- 
trodynamometer.  One  coil  F 
in  this  instrument  is  fastened  to 
the  frame  of  the  instrument,  and 
the  other  coil  M,  which  stands 
at  right  angles  to  the  first,  is 
suspended  by  a  heavy  silk  fibre 
so  that  it  is  free  to  move.  The 
end  of  the  wire  composing  the 
movable  coil  dips  into  little  cups 
CC,  containing  mercury,  which 
are  connected  with  a  circuit  so 
that  the  current  can  enter  and 
leave  the  coil.  The  movable 
coil  is  attached  to  a  spring  G, 
the  other  end  of  which  is  con- 
nected to  a  thumbscrew  T9 
called  a  Torsion  Head,  by  means 

of  which  this  spring  may  be  twisted.  When  a  current-flows  in  the  coils, 
the  magnetic  force  tends  to  turn  the  movable  coil  around  so  as  to  place 
it  parallel  with  the  fixed  coil.1  This  force  is  balanced  by  twisting  the 
spring  by  means  of  the  thumbscrew.  The  amount  of  twist  as  shown  by  a 
pointer  B,  attached  to  the  screw,  is  proportional  to  the  force  exerted 
by  the  coils  on  each  other  •  and  this  force  is  proportional  to  the  square 

1  Article  143. 


FlG.  99. —  Siemens  Electrodynamometer. 


188 


ELECTRICITY   AND   MAGNETISM 


of  the  current  flowing  in  the  coil,  since  the  magnetism  set  up  by  each 
coil  is  proportional  to  the  current,  and  they  act  on  each  other  mutually. 
The  pointer  N  indicates  whether  the  movable  coil  is  at  its  zero  posi- 
tion. The  "binding  posts  "  for  connecting  the  instrument  into  the  cir- 
cuit are  shown  at  AA. 

177.  The  Kelvin  Balance.  —  Very  accurate  and  permanent  stand- 
ard instruments  for  measuring  currents  by  their  direct  magnetic 
action  have  been  designed,  but  they  have  not  been  made  sufficiently 
portable  to  bring  them  into  much  use.  The  most  important  of  these 


FIG.  ioo.  —  Kelvin  Balance. 

are  the  current  balances  of  Lord  Kelvin,  formerly  Sir  William  Thomson, 
one  of  which  is  illustrated  in  Figure  ioo.  The  fixed  and  movable  coils  in 
these  instruments  are  parallel  to  each  other  and  horizontal.  The  force 
with  which  the  coils  tend  to  move  toward  each  other  when  a  current 
flows  in  them  is  directly  balanced  and  weighed  by  means  of  a  slider 
moving  on  a  scale  beam.  In  order  to  avoid  any  effect  from  the  earth's 
magnetism,  coils  are  placed  at  both  ends  of  the  balance  arm  and  are 
electrically  connected  so  that  the  magnetic  force  of  the  two  sets  of  coils 
tends  to  tip  the  beam  in  the  same  direction,  but  the  effect  of  the  earth's 
magnetism  on  the  two  swinging  coils  is  balanced. 


MEASUREMENT  OF  CURRENTS   AND   PRESSURES 


189 


178.  Hot  Wire  Amperemeters.  —  Instruments  utilizing   the    heating 
effect  of  the  current  are  usually  called  Hot  Wire  Instruments.     If  the 
heated  wire  is  carefully  enclosed  so  that  its  temperature  is  not  affected 
by  air  currents,  it  will  rise  to  a  definite  number  of  degrees  in  temperature 
for  every  current  that  is  passed  through  it,  and  the  rise  is  proportional 
to  the  square  of  the  current.     The  length  of  the  wire  increases  practi- 
cally in  direct  proportion  to  its  rise  in  temperature  when  it  is  heated, 
and  the  length  again  decreases  when  the  wire  is  cooled.     Consequently, 
when  currents  of  different  strengths  flow  through  a  wire  it  will  take  up  a 
corresponding  length  with  each  current,  and  measuring  its  length  there- 
fore measures  the  square  of  the  current.     A  simple  form  of  ampere- 
meter depending  on  this  action  is  shown  in  Figure  101.     A  long,  thin  wire 
is  clasped  at  one  end 

in  a  stationary  bind- 
ing post,  and  the  other 
end  is  wrapped  around 
and  fastened  to  a  small 
wheel  of  metal.  This 
wheel  is  supported  on 
steel  pivots,  one  of 
which  is  connected  to 

another  binding  post.  The  wire  is  kept  under  a  constant  strain  by 
means  of  a  spring,  which  is  also  fastened  to  the  periphery  of  the  wheel. 
When  the  wire  is  heated  and  lengthens,  the  wheel  is  turned  by  the  con- 
traction of  the  spring,  and  when  the  wire  is  again  cooled  and  contracts 
it  pulls  the  wheel  back  to  its  old  position.  The  wheel  carries  a  pointer, 
which  moves  over  a  graduated  scale,  so  that  the  position  of  the  wheel 
may  be  quickly  seen  when  any  current  flows  in  the  wire. 

179.  Amperemeter  Scales.  —  Many  amperemeters  have  scales  that  are 
uniformly  graduated,   and    the   readings    can  -  only   be    converted   into 
amperes  by  consulting  a  calibration  curve  l  or  a  table  giving  the  values 
of  different  readings  in  amperes.     Other  instruments  are  constructed  so 
that  the  readings  may  be  multiplied  by  a  fixed  constant  which  has  been 
experimentally  determined,  for   the   purpose  of  converting   them   into 
amperes.     In  still  other  instruments,  which  are  said  to  be  Direct  Read- 

1  Article  153. 


FIG.  101.  — Simple  Hot  Wire  Amperemeter. 


ELECTRICITY  AND    MAGNETISM 


ing,  the  scales  are  so  divided  and  marked  that  the  divisions  read  directly 
in  amperes.  It  is  needless  to  say  that  direct  reading  instruments  are  the 
best  and  most  convenient  for  use. 

180.  Alternating  Current  Amperemeters.  —  Currents  which  rapidly 
alternate  in  direction,  as  do  the  currents  of  many  electric  light  plants, 
cannot  be  measured  by  magnetic  instruments  having  permanent  magnets, 
since  the  tendency  of  such  currents  is  to  first  deflect  the  moving  parts 
in  one  direction  and  then  in  the  other  and  the  pointer  stands  still  or 
nearly  so.  Such  currents  can  be  measured  by  magnetic  instruments  of 


FIG.  102.  —  Plan  of  Thomson  Alternating  Current  Amperemeter. 

the  first  class  because  the  soft  iron  core  is  a/ways  attracted  by  a  coil  in 
which  a  current  flows  without  regard  to  the  direction  of  the  current. 
The  iron  cores  in  instruments  designed  to  measure  these  Alternating 
Currents  must  be  built  up  from  thin  strips  or  fine  iron  wires,  so  that 
currents  shall  not  be  set  up  in  them  by  the  reversals  of  the  magnetism.1 
An  instrument  in  which  a  thin  strip  or  disk  of  iron  is  used  is  commonly 
called  a  "  Magnetic  Vane  "  instrument.  One  of  this  type  is  illustrated, 

139. 


MEASUREMENT   OF  CURRENTS  AND   PRESSURES  191 

with  its  cover  taken  off  so  as  to  expose  the  working  parts,  in  Figure 
102.  The  parts  of  this  instrument  are  indicated  by  the  letters,  where 
D  is  the  current  coil,  C  the  thin  movable  iron  vane,  B  the  needle, 
S  the  scale,  and  AA  the  binding  posts  which  are  connected  to  the  coil 
D  by  the  wires  WW. 

Electrodynamometers  and  other  instruments  which  depend  for  their 
indications  upon  the  mutual  attractions  of  two  coils  may  be  used  to 
measure  alternating  currents,  because  the  current  reverses  in  the  two 
coils  at  the  same  instant,  and  the  magnetic  attraction  between  the  coils 
is,  therefore,  always  in  the  same  direction.  The  heating  effect  of  cur- 
rents is  independent  of  their  direction,  so  that  hot  wire  instruments 
may  also  be  used  to  measure  alternating  currents. 

181.  Shunted  Amperemeters.  —  When  very  large  currents  are  to  be 
measured,  it  is  often  inconvenient  and  expensive  to  build  an  ampere- 
meter of  sufficient  capacity  for  the  purpose.     In  this  case  an  ampere- 
meter of  small  capacity  may  be  shunted  by  a  copper  or  German  silver 
wire  or  rod,  and  the  shunted  instrument  may  then  be  calibrated  and 
used  to  measure  the  large  current.     This  arrangement  has  become  quite 
universal  in  the  large  electric  light  works  where  very  great  currents  are 
to  be  measured,  and  it  is  not  uncommon  in  ordinary  portable  instruments. 
For  instance,  nearly  all  Weston  amperemeters  consist  of  a  milliampere- 
meter  arranged  with  a  proper  shunt  inside  the  case,  as  illustrated  in 
Figure  97,  so  that  the  desired  range  is  obtained. 

182.  Voltmeters.  —  The  commonest  method  of  measuring  an  electric 
pressure  is  to  measure  the  current  which  it  causes  to  pass  through  a 
known  high  resistance.     This  resistance  may  be  connected  permanently 
in  the  circuit  of  a  sensitive  amperemeter,  such  as  a  milliamperemeter, 
and  the  instrument  may  be  calibrated  so  that  its  indications  may  be 
readily  converted  into  volts. 

Instruments  that  are  used  for  everyday  measurements  of  electric  pres- 
sures are  called  Voltmeters,  because  they  measure  volts.  By  properly 
dividing  the  scale  upon  which  the  indications  are  made,  voltmeters  may 
be  made  direct  reading.  Figure  103  shows  a  Weston  direct  reading  volt- 
meter, in  which  the  working  parts  are  similar  to  those  of  the  ampere- 
meter shown  in  Figure  97,  but  in  the  voltmeter  a  high  resistance  spool 
of  fine  wire  is  placed  in  series  with  the  D'Arsonval  galvanometer  coil, 


ELECTRICITY   AND    MAGNETISM 


instead  of  a  low  resistance  shunt  being  placed  in  parallel  with  it,  as  is 
done  with  the  amperemeter. 

The  voltmeter  that  is  shown  in  the  figure  has  three  binding  posts. 
The  instrument  is  connected  to  a  circuit  by  binding  posts  AA'  for  ordi- 
nary use.  Then,  when  the  key  K  is  closed,  current  flows  through  the 
high  resistance  coil  E  and  the  movable  coil  C,  and  the  pointer  is  de- 
flected a  distance  proportional  to  the  electrical  pressure  of  the  circuit. 
When  the  instrument  is  connected  to  the  circuit  by  means  of  the  binding 


FIG.  103.  —  Plan  of  Weston  Voltmeter. 

posts  Aa,  and  the  key  is  closed,  the  current  flows  through  the  coil  e  and 
thence  through  the  movable  coil  C.  The  coil  e  is  adjusted  so  that  the 
resistance  between  binding  posts  Aa  is  just  one-twentieth  of  the  resist- 
ance between  binding  posts  AA\  and  the  instrument  is  therefore  twenty 
times  more  sensitive  where  the  binding  posts  Aa  are  used.  If  two  volts 
cause  a  movement  of  the  pointer  a  certain  distance  across  the  scale  in 
one  case,  it  requires  forty  volts  to  cause  an  equal  movement  in  the  other 
case.  Many  voltmeters  are  made  with  only  one  resistance  coil  and  only 


MEASUREMENT  OF  CURRENTS  AND   PRESSURES 


193 


one  scale.     The  letters  in  this  figure  refer  to  the  same  parts  as  similar 
letters  in  Figures  97  and  98. 

Voltmeters  are  also  made  upon  the  same  principle  as  the  amperemeter 
shown  in  Figure  96,  but  the  coil  is  wound  with  many  turns  of  fine  wire, 
making  a  high  resistance,  instead  of  being  made  with  a  few  turns  of 
coarse  wire.  This  form  of  voltmeter  has  the  same  fault  as  the  ampere- 
meter of  the  same  class  —  that  of  being  inaccurate ;  and  it,  therefore, 
is  not  as  satisfactory  for  use  in  many  places  as  more  accurate  instruments 
made  with  very  little  or  no  iron  in  their  working  parts. 

In  electric  light  plants  where  current  is  produced  for  use  in  incandes- 
cent lamps,  it  is  very  important  that  the  pressure  be  kept  as  closely  as 
possible  to  the  exact  pressure  with  which  the  lamps 
were  designed  to  be  used.  Consequently,  in  such 
places  the  most  accurate  and  reliable  voltmeters  or 
Pressure  Indicators,  as  they  are  sometimes  called,  are 
needed. 

Voltmeters  of  the  type  just  described  are  usually 
made  with  a  very  high  resistance,  so  that  only  a  small 
current  flows  through  them,  and  they  therefore  may 
be  used  without  causing  an  appreciable  change  of  the 
current  in  a  circuit. 

Figure  104  shows  a  hot  wire  voltmeter,  which  is 
called  after  its  inventor,  Cardew.  This  was  largely 
used  at  one  time  to  measure  alternating  electric 
pressures,  and  it  is  still  used  to  a  considerable  ex- 
tent in  England  for  that  purpose.  The  indications 
of  this  instrument  are  dependent  upon  the  expansion 
of  a  very  fine  platinum-silver  wire  (y^wo  mc^  m 
diameter),  through  which  the  current  passes.  This 
wire  is  from  8  to  12  feet  long,  and  of  such  high 
resistance  per  foot  that  its  resistance  alone  is  sufficient 
for  use  up  to  a  pressure  of  120  volts,  but  another  re- 
sistance coil  is  put  in  series  with  the  instrument  when  it  is  used  to 
measure  high  pressures. 

Voltmeters  are  used  for  measuring  pressures,  and  therefore  are  not 
intended  to  be  connected  in  series  in  a  circuit.  They  are  designed  to 


FlG.  104.  —  Cardew 
Voltmeter. 


194 


ELECTRICITY   AND    MAGNETISM 


be  connected  between  the  points  whose  difference  of  pressure  it  is 
desired  to  measure. 

183.    Electrostatic  Voltmeters.  —  Another  entirely  distinct  method  of 
measuring  electric  pressures  is  by  means  of  electrometers.     In  Article  13 

it  is  explained  that  elec- 
trometers are  instruments 
for  determining  the  quantity 
of  electricity  on  a  charged 
body,  by  measuring  its  at- 
traction for  another  charged 
body.  It  is  also  explained 
in  Article  17  that  electric- 
ity at  rest  at  a  high  pres- 
sure constitutes  a  positive 
charge,  and  electricity  at 
rest  at  a  low  pressure  con- 
stitutes a  negative  charge. 
It  is  a  fact  that  the  terms 
positive  and  negative  charge 
must  be  taken  as  relative 
terms,  exactly  as  are  the 
terms  high  and  low  press- 
ure. An  electrometer  is 
an  instrument  by  means  of 
which  the  attraction  be- 
tween two  charges  may  be 
measured. 

One   form  of   electrom- 
eter   is    shown    in    Figure 

105.  In  this  electrometer  there  is  a  needle,  made  of  aluminum,  and  a 
sort  of  metal  pill  box,  cut  into  quadrants  (quarters}.  If  the  opposite 
quarters  are  connected  together,  as  shown  in  the  figure,  and  one  pair 
of  quarters  are  connected  to  the  needle,  the  needle  tends  to  be  deflected 
by  the  attraction  and  repulsion  of  the  charges,  when  a  charge  of  one  sign 
is  communicated  to  the  needle  and  its  connected  pair  of  quadrants,  and 
a  charge  of  the  opposite  sign  to  the  other  pair  of  quadrants.  The  force 


FIG.  105.  —  Quadrant  Electrometer. 


MEASUREMENT  OF  CURRENTS  AND   PRESSURES 


195 


FIG.  1050.  —  Plan  of  Quad- 
rants and  Needle  for  a 
Quadrant  Electrometer. 


with  which  the  needle  tends  to  turn  may  be  measured  by  a  torsion 
head,  as  in  an  electrodynamometer,  or  by  suspending  the  needle  so 
that  a  certain  portion  of  its  weight  must  be 
lifted  as  it  turns. 

If  the  two  poles  of  a  battery  are  connected 
to  the  two  terminals  of  the  electrometer,  one 
terminal  of  the  instrument  is  brought  to  a  high 
pressure  and  the  other  to  a  low  pressure,  on 
account  of  the  action  of  the  battery,  and  they, 
therefore,  hold  corresponding  positive  and  neg- 
ative charges.  The  deflection  of  the  needle 
indicates  the  pressure  developed  by  the  battery. 
This  pressure  may  be  directly  read  off  in  volts, 
if  the  instrument  has  been  properly  calibrated. 
In  the  same  way,  if  the  two  ends  of  a  resistance  through  which  a  current 
is  flowing,  such  as  an  electric  lamp,  are  connected  to  the  electrome- 
ter, one  terminal  of  the  instrument  is  brought 
to  a  high  and  the  other  to  a  low  pressure, 
and  the  deflection  of  the  needle  shows  the 
difference  of  pressure  between  the  ends  of  the 
resistance. 

Electrometers  made  for  use  in  everyday  meas- 
urements of  electric  pressure  are  usually  called 
Electrostatic  Voltmeters.  They  are  particularly 
useful  for  measuring  alternating  electric  press- 
ures, since  the  polarity  of  the  two  pairs  of 
quadrants  and  of  the  needle  change  at  the  same 
instant,  and  consequently  the  needle  is  deflected 
continuously  in  the  same  direction.  Figure  106 
shows  such  a  voltmeter,  arranged  for  use  in  light 
and  power  stations.  It  is  seen  from  the  figure 
that  there  are  a  large  number  of  sets  of  quad- 
rants and  needles,  one  above  the  other.  This  is 
to  make  the  tendency  to  deflect  stronger.  Fig- 
ure 107  shows  an  electrostatic  voltmeter  made  for  measuring  pressures 
of  several  thousand  volts. 


FIG.  106.  —  Electrostatic 
Voltmeter. 


IQ6 


ELECTRICITY   AND    MAGNETISM 


184.  Measuring  Pressures  by  Comparison  with  Standard.  —  Still 
another  method  of  measuring  an  electric  pressure  is  to  compare  it 
with  a  standard  pressure.  If  a  known  large  resistance  is  connected 

between  the  points  whose  difference 
of  pressure  it  is  desired  to  measure,  a 
small  current  will  flow  through  the  re- 
sistance, and  the  pressure  will  fall 
along  the  path  of  the  current  in  pro- 
portion to  the  resistance  passed  over. 
Now  suppose  the  terminals  of  a  battery 
cell  £  (Fig.  108)  are  connected  in 
series  with  a  galvanometer  G  to  certain 
points  on  the  resistance,  in  such  a  way 
that  the  pressure  of  the  cell  is  in 
opposition  to  the  difference  of  pressure 
between  the  points.  If  the  latter  press- 
ure is  greater  than  that  of  the  cell,  a 
current  will  flow  through  the  cell  and 
galvanometer,  and  the  galvanometer 
needle  will  be  deflected.  The  same  thing  will  occur  if  the  pressure  of 
the  cell  is  the  greater,  but  the  current  and  the  galvanometer  deflection 
which  it  produces  will  be  reversed. 

Finally,  if  the  portion  of  the  resistance  which  is  between  the  terminal 
connections  of  the  cell  is 

so  adjusted  that  no  cur-     ||  I 

rent    flows     through     the  10,000  OHMS  TOTAL 

galvanometer,  the  fall  of 
pressure  through  that  part 
of  the  resistance  exactly 
equals  the  pressure  pro- 
duced by  the  cell.  The 
total  pressure  to  be  meas- 
ured is  then  equal  to  the 

pressure  developed  by  the  cell,  multiplied  by  the  ratio  of  the  total  resist- 
ance to  the  balancing  resistance.  In  the  figure,  the  pressure  of  the 
cell  is  marked  1.2  volts,  the  total  resistance  is  10,000  ohms,  and  the 


FIG.  107.  —  Electrostatic  Voltmeter  for 
indicating  High  Pressures. 


G  S  =  1.2  VOLTS 

FIG.  108.  —  Measuring  Pressure  by  Comparison. 


MEASUREMENT  OF  CURRENTS   AND  PRESSURES 


197 


balancing  resistance  is  100  ohms.     Assuming  a  balance,  the  total  press- 
ure must  be  1.2  x  1 0,000 -f-  100  =  120  volts. 

185.  Potentiometers.  —  A  special  arrangement  for  measuring  press- 
ures by  comparison  is  often  called  a  Potentiometer,  and  the  cells  used 
for  the  comparison  are  called  Standard  Cells.  It  is  evident  that  stand- 
ard cells  must  develop  a  very  uniform  pressure  under  all  conditions 
of  their  use.  The  best  standard  cell  is  that  called  Clark's  Cell,  after  its 
inventor.  This  was  recommended  by  the  Chicago  Electrical  Congress 
to  be  used  as  a  comparative  standard  of  pressures,  and  its  pressure  was 


VOLTMETER 

FIG.  109.  —  Measurement  of  Current  by  Voltmeter  and  Known  Resistance. 

given  in  accordance  with  experimental  tests  to  be  1.434  volts  at  15° 
centigrade,  when  set  up  according  to  fixed  instructions.  Professor 
Carhart  and  others  have  endeavored  to  make  a  standard  cell  with 
exactly  one  volt  pressure. 

Portable  voltmeters  have  been  made  upon  the  principle  of  a  potenti- 
ometer, but  without  much  success. 

186.  Indirect  Measurement  of  Current.  —  Electric  currents  may  be 
indirectly  measured  by  means  of  a  known  resistance,  placed  in  the 
circuit  through  which  the  current  flows,  and  a  voltmeter  connected 
across  its  terminals.  In  this  case,  the  voltmeter  is  used  to  measure  the 


198  ELECTRICITY   AND    MAGNETISM 

difference  of  pressure  between  the  ends  of  the  resistance  (Fig.  109), 
and  the  current  may  be  at  once  calculated  from  Ohm's  Law.  This 
process,  however,  simply  amounts  to  an  indirect  application  of  the 
principle  of  a  shunted  amperemeter!  It  must  be  remembered  that  an 
electromagnetic  voltmeter  is  no  more  than  a  milliamperemeter  which 
is  graduated  to  read  in  volts. 

QUESTIONS 

1.  What  three  effects  of  electricity  can  be  made  use  of  in  measuring  electric 
currents? 

2.  Of  what  service  have  voltameters  proved  in  ordinary  current  measurements? 

3.  What  is  an  amperemeter? 

4.  What  is  an  amperemeter  used  for? 

5.  What  is  a  microampere? 

6.  Into  what  three  general  classes  may  amperemeters  be  divided? 

7.  Compare  the   second  class  of  amperemeters  with  D'Arsonval  and  ordinary 
reflecting  galvanometers. 

8.  Why  must  an  opposing  spring,  or  some  other  arrangement  of  that  kind,  be 
used  against  the  force  of  the  current  tending  to  cause  movement  in  the  ampere- 
meter? 

9.  Describe  a  Weston  amperemeter. 

10.  What  is  an  electrodynamometer? 

11.  Describe  a  Siemens  electrodynamometer. 

12.  Why  is  the  force  acting  in  an  electrodynamometer  proportional  to  the  square 
of  the  current? 

13.  Describe  a  Kelvin  balance. 

14.  How  is  the  effect  of  the  earth's  magnetism  overcome  in  a  Kelvin  balance? 

15.  What  is  the  principle  of  hot  wire  instruments? 

1 6.  Why  do  the  needles  of  hot  wire  amperemeters  move  in  proportion  to  the 
squares  of  the  currents? 

17.  Describe  a  hot  wire  instrument. 

1 8.  What  is  a  direct  reading  amperemeter? 

19.  Why  cannot  instruments  containing  permanent  magnets  be  used  to  measure 
alternating  current? 

20.  Why  can  instruments  with  finely  divided  soft  iron  cores  be  used  in  measuring 
alternating  currents? 

21.  Why  can  electrodynamometers  and  hot  wire  instruments  be  used   in  meas- 
uring alternating  currents? 

22.  What  are  shunted  amperemeters? 

23.  If  an  amperemeter  of  100  milliamperes  range  of  scale  is  to  be  used  with  a 


MEASUREMENT  OF  CURRENTS   AND   PRESSURES  199 

shunt  to  measure  100  amperes,  what  must  be  the  relative  resistance  of  amperemeter 
and  shunt? 

24.  What  is  a  voltmeter? 

25.  What  is  the  principle  of  a  voltmeter? 

26.  What  is  the  difference  between  a  voltmeter  and  an  amperemeter? 

27.  How  can  a  milliamperemeter,  with  a  large  resistance  coil  in  series  with  it, 
be  calibrated  so  that  it  will  be  a  direct  reading  voltmeter  ? 

28.  Are  all  voltmeters,  depending  upon  magnetic  attraction,  really  amperemeters 
calibrated  to  read  volts  ? 

29.  How  is  a  hot  wire  amperemeter  modified  so  that  it  may  be  used  as  a  voltmeter? 

30.  What  is  an  electrometer? 

31.  What  is  an  electrostatic  voltmeter? 

32.  Why  does  the  needle  of  an  electrostatic  voltmeter  tend  to  move? 

33.  What  is  a  potentiometer? 

34.  What  battery  cell  did  the  Chicago  Electrical  Congress  adopt  as  a  standard 
of  pressure  ? 

35.  Why  will  the  galvanometer  needle    of  a  potentiometer   show  no  deflection 
when  the  fall  of  pressure  in  the  resistance,  across  which  the  standard  cell  is  con- 
nected, is  equal  to  the  pressure  of  the  cell? 

36.  If  the  resistance  included  between  the  terminals  of  the  standard  cell  of  a 
potentiometer  is  one-tenth  of  the  total  resistance,  how  much  greater  is  the  pressure 
to  be  measured  than  that  of  the  cell? 

37.  What  would  happen  if  an  amperemeter  were  connected  in  parallel  across  a 
circuit  instead  of  in  series  with  the  circuit? 

38.  What  would  happen  if  a  voltmeter  were  connected  in  series  with  a  circuit,  as 
an   amperemeter   should    be,  instead  of  between  the  terminals  of  the  circuit? 

39.  If  a  100  ampere  amperemeter,  with  a  resistance    of  one-thousandth    of  an 
ohm,  should  be  connected  between  two  points  in  a  circuit  having  a  difference  of 
pressure  of  100  volts,  how  much  current  would  instantly  pass  through  the  ampere- 
meter?    What  would  be  the  result  ? 


CHAPTER  XIV 

MEASUREMENT  OF  ELECTRICAL  POWER. 
CONDENSERS,  AND  MEASUREMENT  OF  CAPACITY 

187.  Measurement  of  Electric  Power;  Wattmeter.  —  The  electric 
power  which  is  used  in  any  part  of  a  continuous  current  circuit  may  be 
determined  by  measuring  the  current  flowing  by  an  amperemeter,  and 
the  pressure,  or  Voltage  as  it  is  often  called,  at  the  terminals  of  the 
portion  of  the  circuit,  by  a  voltmeter.  The  values  of  these  being  multi- 
plied together  give  the  power  in  watts.1  Instruments  are  made  in 
which  the  double  measurement  and  multiplication  are  all  made  together, 
so  that  their  indications  are  directly  proportional  to  power.  Such 
instruments  are  called  Wattmeters,  because  they  measure  watts. 

The  simplest  wattmeter  is  a  form  of  electrodynamometer,  in  which 
one  coil  is  wound  with  many  turns  of  fine  wire  exactly  as  though  it  were 
to  be  used  as  a  voltmeter  coil,  and  the  other  coil  is  wound  with  a  few 
turns  of  coarse  wire  as  though  it  were  to  be  used  in  an  amperemeter. 
For  convenience  we  will  call  the  two  coils  respectively  the  pressure  coil 
and  the  current  coil.  The  action  of  such  a  wattmeter  is  best  explained 
by  an  illustration.  Suppose  it  is  desired  to  measure  the  power  used  by 
an  electric  motor,  then  the  current  coil  of  the  wattmeter  is  connected 
in  series  with  the  motor,  and  the  pressure  coil  is  connected  across  the 
terminals  of  the  motor.  The  magnetic  effect  of  the  current  coil  is  then 
proportional  to  the  current  which  flows  through  the  motor,  and  the 
magnetic  effect  of  the  pressure  coil  is  proportional  to  the  pressure  at 
which  the  current  is  supplied  to  the  motor.  The  indications  of  an 
electrodynamometer  are  proportional  to  the  product  of  the  magnetic 
effects  of  the  two  coils.2  Consequently,  in  this  case  the  indications  are 
proportional  to  current  times  pressure,  or  watts,  instead  of  current  times 
current  as  in  the  Siemens  electrodynamometer.  Figure  109^  shows  a 
portable  form  of  wattmeter  with  its  cover  removed  so  as  to  show  the 

1  Article  no.  2  Article  176. 

20C 


MEASUREMENT  OF   ELECTRICAL   POWER 


2O I 


working  parts,  which  are  indicated  by  letters.  MM  and  OO  are  binding 
posts,  the  first  of  which  are  terminals  for  the  "  series  coil,"  and  the 
second  are  terminals  for  the  "  pressure  coil "  (one  of  the  latter  is  at 
the  far  corner  of  the  instrument  and  is  hidden)  ;  A  is  the  stationary  or 
fixed  coil ;  BB  is  the  movable  coil ;  P  is  the  pointer  or  "  needle  "  ;  H  is 
the  scale ;  K  is  an  extra  resistance  which  is  placed  in  series  with  the 
pressure  coil ;  D  is  a  torsion  head  by  means  of  which  the  spring  C  may 
be  turned  so  as  to  bring  the  movable  coil,  which  is  attached  to  it,  into 
the  zero  position.  This  position  is  indicated  by  the  pointer  P'.  When 


FIG.  toga.  —  Partial  Perspective  View  of  Hoyt  Wattmeter. 

the  pointer  P'  points  to  zero,  the  pointer  /'which  is  attached  to  the 
torsion  head  points  to  the  reading  of  the  instrument.  R  and  Q  are  the 
wooden  base  and  supports  of  the  instrument. 

Wattmeters  are  often  made  so  that  the  needle  P  is  directly  attached 
to  the  movable  coil  and  one  end  of  the  spring  is  attached  to  the  frame 
of  the  instrument  (instead  of  to  a  torsion  head),  while  the  other  end  is 
attached  to  the  coil.  Then  the  reading  of  the  instrument  depends  on  the 
amount  of  motion  of  the  coil,  just  as  was  explained  in  Article  175 
with  respect  to  the  Weston  amperemeter. 

Wattmeters  may  be  calibrated  by  comparing  their  readings,  when  con- 
nected to  a  continuous  current  circuit,  with  the  indications  of  standard 
voltmeters  and  amperemeters.  By  proper  construction  and  adjustment 


202 


ELECTRICITY   AND   MAGNETISM 


of  their  scales  they  may  be  made  direct  reading.     Figure  109^  shows 
the  way  in  which  a  wattmeter  is  connected  to  a  circuit. 


FIG.  109/5.  —  Diagrammatic  Illustration  of  Wattmeter,  W,  connected  to  a  Circuit  for  the 
Purpose  of  measuring  the  Power  delivered  to  Incandescent  Lamps  or  Other  Ap- 
paratus. CC',  VV  are  the  binding  posts  and  LLL  are  the  lamps. 

It  is  also  possible  to  make  electrostatic  wattmeters,  and  wattmeters 
based  upon  other  principles. 

188.    Recording  Wattmeters.  —  Recording  wattmeters  may  be  used 
to  show  the  amount  of  power  used  each  month  by  the  customers  of 

electric  plants.  One  of 
the  commonest  forms  of 
wattmeters  used  for  this 
purpose  is  that  shown  in 
Figure  no,  known  as  the 
Thomson  recording  watt- 
meter, after  its  inventor. 
This  is  built  like  a  little 
electric  motor  without 
any  iron  in  its  working 
parts.  It  is  arranged 
with  its  revolving  part  or 
Armature  A  as  a  pressure 
coil,  and  its  magnetizing 
windings  W  W  as  a  cur- 
rent coil.  The  magnetic 
pull  which  tends  to  make  the  armature  rotate  is  proportional  to  the 
product  of  the  two  magnetizing  effects,  and  this  is  proportional  to  the 
watts  in  the  circuit,  exactly  as  in  an  electrodynamometer. 

If  the  speed  of  such  an  armature  is  made  to  be  proportional  to  the 


FIG.  no.  —  Interior  of  Thomson  Recording  Wattmeter, 


MEASUREMENT  OF  ELECTRICAL  POWER  2O3 

magnetic  pull,  it  is  easily  seen  that  every  revolution  of  the  armature 
means  a  certain  number  of  watts  used  for  a  fixed  length  of  time.  Such 
instruments  usually  have  attached  to  the  spindle  of  the  armature  a  set  of 
dials  D  like  those  of  a  gas  meter,  which  record  the  revolutions  and  are 
so  marked  that  the  consumption  of  electric  energy  may  be  recorded  in 
Watt  Hours.  Watt  hours  are  the  product  of  the  number  of  watts  by  the 
number  of  hours  during  which  the  power  is  used.  Since  the  dials  record 
a  total  number  of  watt  hours  which  are  added  together,  or  "integrated," 
by  a  meter  during  the  period  that  it  operates,  these  meters  are  properly 
called  Integrating  Wattmeters. 

If  no  external  retarding  force  were  applied  to  the  armature  cf  such  an 
instrument,  it  would  run  away  as  soon  as  placed  in  service,  and  in  order 
that  its  speed  may  be  proportional  to  the  watts  the  retarding  force  must 
be  in  proportion  to  the  speed.  This  is  very  ingeniously  arranged  in  the 
Thomson  recording  wattmeter  by  placing  at  the  bottom  of  the  spindle  S 
a  flat  disk  of  copper  C,  on  either  side  of  which  are  placed  the  poles  of 
permanent  magnets  M.  The  rotation  of  the  disk  between  the  magnet 
poles  generates  electric  currents  in  it  which  are  attracted  by  the  magnets 
and  retard  the  motion  of  the  disk. 

Some  other  meters  for  use  in  determining  the  amount  of  power  con- 
sumed by  customers,  which  are  externally  similar  to  the  Thomson,  read 
Ampere  Hours,  instead  of  watt  hours.  An  ampere  hour  is  equal  to  3600 
Ampere  Seconds,  but  one  ampere  second  (that  is,  one  ampere  flowing 
for  one  second)  means  the  transfer  of  one  coulomb  of  electricity  through 
the  circuit.  Consequently,  the  readings  of  meters  which  record  in 
ampere  hours  are  directly  comparable  with  the  indications  of  the  Edison 
electrolytic  meter  which  has  been  mentioned  before.1  Meters  which 
read  in  ampere  hours  are  sometimes  called  Coulomb  Meters. 

The  reading  of  ampere  hours  has  no  relation  to  the  power  consumed 
in  a  circuit  unless  the  pressure  in  the  circuit  is  known,  but  in  the  cases 
where  such  meters  are  used  the  pressure  is  intended  to  be  kept  at  a 
constant  known  value,  so  that  the  watt  hours  used  by  each  customer  may 
be  easily  determined.  This  is  done  by  multiplying  the  ampere  hours 
reading  of  his  meter  by  the  constant  pressure  in  the  circuit. 

The  wattmeters  which  have  been  described  here  are  also  satisfactory 
1  Articles  159  and  172. 


204  ELECTRICITY  AND   MAGNETISM 

for  use  in  alternating  current  circuits,  though  the  relations  in  such 
circuits  which  exist  between  current,  pressure,  and  power  are  not  so 
simple  as  in  direct  current  circuits.  The  important  and  interesting 
features  of  alternating  current  circuits  are  treated  in  succeeding  chapters. 

189.  Measurement  of  the  Pressure  of  a  Static  Charge.  —  The  elec- 
trical  pressure    of  a   conductor   carrying   a   charge    of    electricity    is 
ordinarily  reckoned  as  the   difference   between  its   potential   and  the 
average  electrical  pressure  of  the  earth's  surface,  which  is  called  zero. 
This  is  similar  to  the  reference  of  levels  or  heights  to  the  sea  level  as 
a  zero  point  from  which  to  start.1     The  'electrical  pressure  of  a  charged 
conductor   cannot   be   measured   by  an   ordinary  voltmeter,  since  the 
charge  would  be  at  once  dissipated  by  the  current  which  would    flow 
through  the  voltmeter  when  connected  between  the  conductor  and  the 
earth.      The  same  is  true  of  condensers  in  all  of  their  forms.     The 
pressure  may,  however,  be  measured  by  a  sufficiently  sensitive  electrom- 
eter or  electrostatic  voltmeter.     For  instance,  in  the  case  of  a  quadrant 
electrometer  which  is  briefly  described  in  Article  183,  the  needle  and  its 
pair  of  quadrants  may  be  connected  to  earth  and  the  other  pair  of  quad- 
rants to  the  charged  body.     Then,  if  the  instrument  is  sufficiently  sensi- 
tive, the  needle  will  be  deflected  an  amount  which  is  proportional  to  the 
difference  between  the  earth's  electrical  pressure  and  that  of  the  charged 
body,  or  between  the  two  plates  of  the  condenser. 

190.  Specific   Inductive   Capacity.  —  The   capacity    of  a   condenser 
depends  directly  upon  the  area  of  its  plates,  their  closeness  together,  and 
the  Specific  Inductive  Capacity  of  the  dielectric.2     Different  insulating 
materials  have  very  different  values  as  dielectrics.     The  inductive  action 
seems  to  be  stronger  through  some  materials  than  through  others,  and  it 
is  less  active  through  air  than  through  any  solids  or  liquids.     Conse- 
quently a  condenser  which  has  air  for  a  dielectric  has  less  capacity  than 
one  of  exactly  equal  size  with  a  solid  dielectric.     The  ratio  of  the  capaci- 
ties of  two  such  condensers,  in  which  the  dielectric  of  one  is  air,  is  called 
the  specific  inductive  capacity  of  the  solid  dielectric.     The  annexed  table 
gives  the  approximate  specific  inductive  capacities  of  various  materials. 
That  of  air  is  taken  as  unity  as  a  matter  of  reference,  because  the  inductive 
effect  is  less  through  it  than  through  any  common  substance. 

1  Article  1 7.  2  Article  26. 


CONDENSERS 


205 


MATERIAL 

SPECIFIC  INDUCTIVE 
CAPACITY 

MATERIAL 

SPECIFIC  INDUCTIVE 
CAPACITY 

Air  

I. 

Gutta-percha 

2.C 

Petroleum     .... 

2.1 

Shellac.     .     .     . 

2  Q 

Turpentine    .... 
Rubber     

2.2 
2.7 

Sulphur     .... 
Mica     

3-7 
6.6 

Paraffine  ... 

2.7 

Glass 

5  oo  to  10  oo 

The  table  shows  the  importance  of  carefully  selecting  the  insula- 
tion for  telephone  cables  where  capacity  is  very  objectionable.  In 
fact,  the  insulation  directly  surrounding  the  individual  wires  of  such 
cables  is  often  made  from  crinkled  paper,  so  that  air  -makes  up  a  consider- 
able part  of  the  material  between  the  wires.  While  glass  is  one  of  the 
best  insulators,  it  is  one  of  the  poorest  materials  to  use  for  the  continuous 
insulation  of  wires  in  telephone  cables  on  account  of  its  great  specific 
inductive  capacity. 

Insulated  wires  and  cables  placed  underground  always  have  a  much 
greater  capacity  than  wires  of  the  same  size  and  length  placed  overhead. 
This  is  largely  because  the  dielectric  of  the  underground  wires  is  so  much 
thinner  than  that  of  the  overhead  wires,  and  partially  because  the  induc- 
tive capacity  of  solid  dielectrics  is  greater  than  that  of  air.  The  capacity 
of  an  overhead  wire,  strung  at  a  height  of  30  feet  above  the  ground,  is 
only  about  one-twentieth  of  that  of  a  similar  wire  well  insulated  with  a 
rubber  compound  and  placed  underground,  and  only  about  one-tenth 
of  that  of  a  similar  wire  insulated  with  cotton  and  paraffine  and  placed 
in  a  cable  underground. 

191.  Relation  of  Pressure,  Charge,  and  Capacity  in  a  Condenser. —  It 
was  stated  in  Article  28,  that  a  condenser  of  one  farad  capacity  holds 
opposite  charges  of  one  coulomb  on  its  two  plates  whenever  they  differ 
in  pressure  by  one  volt.  If  one  volt  charges  it  with  two  coulombs,  the 
capacity  is  two  farads ;  if  with  three  coulombs,  three  farads,  and  so  on. 
Or  we  may  say  that  when  the  pressure  is  constant,  the  quantity  of  charge 
is  directly  proportional  to  the  capacity.  Also,  if  the  capacity  is  fixed,  the 
quantity  of  the  charge  is  proportional  to  the  pressure.  These  two  state- 


206  ELECTRICITY   AND    MAGNETISM 

ments  together  show  that  the  electrical  charge  of  a  condenser  varies 
directly  with  the  capacity  and  also  directly  with  the  pressure. 

This  may  be  written  in  symbols,  S'  =  -=,t  where  S'  is  capacity  in  farads, 
Q  quantity  on  each  plate  in  coulombs,  and  E  pressure  in  volts.  Since  a 

microfarad  is  one  millionth  of  a  farad  we  may  also  write  S=  i, 000,000-—, 
where  .S  is  the  capacity  in  microfarads. 

This  relation  may  be  compared  to  the  capacities  of  water  tanks.  If 
a  tank  of  "  unit  capacity  "  is  taken,  or  one  in  which  the  water  has  a 
height  of  one  foot  when  charged  with  one  gallon,  then  the  greater  the 
number  of  gallons  required  to  raise  the  water  level  one  foot  the  greater 
is  the  capacity ;  and  also  with  a  tank  of  given  capacity,  the  quantity  of 
water  in  the  tank  depends  upon  the  height.  The  quantity  of  water  in  a 
tank  therefore  varies-  directly  with  the  "  capacity  "  of  the  tank,  and  also 
directly  with  the  height  that  it  stands  in  the  tank. 

PROBLEMS 

A.  A  condenser  is  charged  with  .00001  of  a  coulomb  of  electricity  when  its  terminals 
are  at  a  difference  of  potential  of  10  volts.    What  is  its  capacity?    Ans.  I  microfarad. 

B.  A  condenser  of  .5  microfarad  capacity  is  charged  by  a  difference  of  potential 
of  100  volts.     What  is  the  quantity  of  the  charge?     Ans.  .00005  coulomb. 

C.  What  pressure  is  required  to  charge  a  .2  microfarad  condenser  with  a  charge 
of  .0001  coulomb?    Ans.  500  volts. 

D.  If  one  condenser  holds  four  times  as  much  electricity  when  charged  by  50  volts 
as  does  another  when  charged  by  100  volts,  what  is  the  ratio  of  the  capacity  of  the 
first  condenser  to  that  of  the  second?     Ans.  8. 

191  a.  Condensers  in  Series  and  Parallel.  —  Since  the  capacity  of  a 
condenser  is  directly  proportional  to  the  area  of  the  plates,1  connecting 
condensers  in  parallel  gives  a  total  or  combined  capacity  which  is  equal 
to  the  sum  of  the  individual  capacities.  Again,  since  the  capacity 
depends  inversely  upon  the  thickness  of  the  dielectric,2  connecting  con- 
densers of  equal  capacity  in  series  gives  a  combined  capacity  equal  to  the 
capacity  of  one  condenser  divided  by  the  number  in  series,  because  con- 
necting condensers  in  series  has  the  effect  of  adding  together  the 
thicknesses  of  the  dielectrics  in  the  different  condensers.  Where 
condensers  of  different  capacities  are  connected  together  in  series,  the  com- 

1  Article  190.  z  Ibid. 


CONDENSERS 


207 


bined  capacity  is  equal  to  the  reciprocal  of  the  sum  of  the  reciprocals  of 
the  individual  capacities.      (  —  =  — | 1 — j*1     Condensers  connected 

in  series  are  sometimes  said  to  be  connected  in  Cascade. 

The  plates  of  standard  condensers  are  usually  made  of  tinfoil,  and  the 
dielectric  of  mica,  paraffined  paper,  or  oiled  paper. 

PROBLEMS 

A.  If  three  |  microfarad  condensers  are  connected  in  parallel,  what  is  their  com- 
bined capacity  ?    Ans.  i^  microfarads. 

B.  Suppose  four  condensers  of  ^,  ^,  I,  and  \\  microfarads  capacity  are  connected 
in  parallel,  what  is  their  combined  capacity  ?    Ans.  3^  microfarads. 

6".    If  the  condensers  of  Example  A  are  connected  in  series,  what  is  their  combined 
capacity  ?     Ans.  \  microfarad. 

D.  If  the  condensers  of  Example  B  are  connected  in  series,  what  is  their  com- 
bined capacity  ?     Ans.  ^  microfarad. 

E.  If  you  have  three  condensers  of  |,  \,  and  I  microfarad  capacity,  respectively, 
what  are  the  various  values  in  microfarads  besides  the  individual  values  that  you  can 
have  at  your  disposal  for  testing  purposes,  using  them  in  all  combinations? 

1st,  Parallels  of  3  and  2.     Ans.    if,   i|,  i£,  f. 

2d,  Series  of  3  and  2.         Ans.   },  ^,  ^,  ^. 

3d,  Series  parallel.  Ans.    i|,  T7Q,  T72,  T65,  f\,  T\. 

192.  Standard  Condensers.  —  Standard  condensers  are  made  of  various 
capacities  and  put  up  in  boxes  so  that  they  may  be  readily  used  for  various 
purposes.  Since  a  capacity  as 
large  as  a  farad  is  really  never 
met  in  the  electrical  industries, 
standard  condensers  are  usu- 
ally made  equal  to  Microfarads2 
(one-millionths  of  'a  farad)  or 
fractions  of  microfarads,  and 
the  microfarad  has  become  the 
common  unit  in  which  capaci- 
ties are  measured.  Figure  1 1 1 
shows  an  adjustable  condenser 

which  is  made  with  five  divi- 

FIG.  in.   Standard  Half  Microfarad  Condenser 
sions    of  .1    microfarad    each.  Box. 

1  Compare  combined  resistances,  Articles  102,  103,  and  104.        2  Article  28. 


208 


ELECTRICITY  AND   MAGNETISM 


The  five  divisions  may  be  put  in  parallel  so  that  the  total  capacity  is  ^ 
microfarad.  Figure  112  illustrates  diagrammatically  the  connections  of 
the  condensers  to  the  brass  blocks  on  top  of  the  box.  Figure  113  shows 
the  way  in  which  the  condensers  are  made  up  of  alternate  plates  of 
tinfoil  and  oiled  paper. 


Hh 


b 

4 


Hh 


§» 


Hh 


FIG.  112.  —  Diagram  of  the  Connections  of  Condenser  Sections  in  Standard  Box. 
£,  5,  S,  are  the  Condenser  Sections  and  B,  B,  B,  are  Brass  Blocks. 

193.  Measurement  of  Capacity  by  Ballistic  Galvanometer.  —  It  is 
very  important  to  make  measurements  of  the  capacity  of  conductors  to 
be  used  in  telephony  and  telegraphy.  This  may  be  done  in  various 

ways,  but  the  method  generally 
used  is  to  compare  the  capacity 
of  the  wire  directly  with  that 
of  a  standard  condenser  by 
means  of  a  Ballistic  Galva- 


nometer. 

A   ballistic  galvanometer   is 
simply  a  sensitive  galvanome- 
ter  which    is    not   dead-beat. 
In  this  case,  if  a  certain  quan- 
tity of  electricity  is  caused  to 
pass  through   the  coils  of  the 
instrument  in  a  very  short  in- 
terval of  time,  its  magnetic  effect  on  the  needle  is  very  much  like  that 
of  a  blow,  while  the  magnetic  effect  of  a  steady  current  is  like  that  of  a 
steady  push  on  the  needle.     The  needle  of  a  galvanometer,  when  such 


FIG.  113. —  Illustration  of  the  Construction  of  a 
Condenser. 


MEASUREMENT  OF  CAPACITY  2OQ 

a  Transient  Current  or  Discharge  passes  through  it,  swings  off  through  an 
angle  which  is  proportional  to  the  quantity  of  electricity  in  the  discharge, 
provided  the  angle  of  swing  or  Throw  is  not  too  great. 

To  measure  the  capacity  of  a  cable,  a  standard  condenser  is  selected 
of  a  capacity  nearly  equal  to  that  of  the  cable.  The  condenser  is 
charged  by  a  battery  of  a  few  cells,  and  by  means  of  a  key  its  connections 
are  then  changed  so  that  it  discharges  through  a  galvanometer.  The 
throw  of  the  galvanometer  needle  (distance  it  swings)  is  observed.  The 
same  battery  is  now  connected  with  one  terminal  to  the  cable  conductor, 
and  its  other  terminal  to  the  cable  sheathing  or  to  the  earth.  In  this  way 
the  cable  is  charged.  The  cable  and  earth  connections  are  then  trans- 
ferred to  the  galvanometer  by  means  of  the  key,  and  the  cable  is  dis- 
charged through  the  galvanometer.  The  throw  of  the  needle  is  again 
observed.  The  two  throws  are  proportional  to  the  quantities  of  elec- 
tricity in  the  charges  of  the  condenser  and  the  cable.  Since  these  were 
charged  by  the  same  battery  and  therefore  to  the  same  pressure,  the 
quantities  of  electricities  are  proportional  to  the  respective  capacities. 
Therefore,  the  capacities  are  proportional  to  the  throws. 

The  object  of  taking  a  condenser  of  a  capacity  nearly  equal  to  that  of 
the  cable  is  to  make  the  throws  nearly  alike,  and  thus  avoid  instrumental 
errors.  When  a  proper  condenser  cannot  be  obtained,  a  shunt  may  be 
applied  to  the  galvanometer,  but  this  is  also  likely  to  introduce  errors 
when  used  with  discharges.  The  insulation  of  the  instruments  and  their 
connections  must  be  as  perfect  as  possible  in  capacity  tests,  as  is  also 
necessary  in  insulation  tests.1 

As  an  example,  suppose  the  discharge  of  a  1  microfarad  condenser 
when  charged  by  five  cells  gives  a  galvanometer  throw  of  200  divisions ; 
and  when  a  cable  two  miles  long  is  charged  by  the  same  cells,  and  dis- 
charged through  the  galvanometer,  the  throw  is  180.  Then  the  capacity 
of  the  cable  is  %%%  x  %  =  .45  microfarads,  and  the  capacity  of  the  cable 
per  mile  is  .45  -5-  2  =  .225  microfarads. 

It  is  usual  to  speak  of  the  "capacity  of  a  wire  "  or  of  a  cable  in  the 
manner  of  the  preceding  paragraphs,  but  it  must  not  be  forgotten  that 
the  conductor  of  the  cable  is  only  one  plate  of  the  condenser.  The 
other  plate  consists  of  the  return  conductor  through  which  the  working 

1  Articles  169  and  170. 
P 


2IO  ELECTRICITY   AND    MAGNETISM 

electric  circuit  is  completed,  such  as  the  earth,  the  cable  sheathing,  or 
another  wire,  while  the  dielectric  of  the  condenser  is  that  intervening 
insulation.  And  the  "  capacity  of  a  cable  "  is  therefore  the  capacity  of 
the  condenser  in  which  the  cable  conductor  constitutes  one  plate  and 
the  insulation  of  the  conductor  constitutes  the  dielectric. 

PROBLEMS 

A.  A  condenser  of  .2  microfarad  capacity,  charged  to  10  volts  difference  of  poten- 
tial, causes  a  throw  of  150  divisions  when  it  is  discharged  through  a  galvanometer. 
A  certain  cable  one  mile  long  causes  a  throw  of  75  divisions  when  charged  by  the  same 
difference  of  potential.     What  is  the  capacity  of  the  cable  ?     Ans.    .1  microfarad. 

B.  A  condenser  of  .3  microfarad  and  a  piece  of  cable  cause  the  same  throw  in  a 
galvanometer  when  the  former  has  been  discharged  after  having  been  charged  by  10 
volts  difference  of  potential  and  the  latter  by  20  volts.     What  is  the  capacity  of  the 
cable?     (Aid:  The  galvanometer  needle  would  be  deflected  only  half  as  far  if  the 
cable  were  charged  to  10  volts.)     Ans.  .15  microfarad. 

C.  An  overhead  telegraph  line,  after  having  been  charged  to  100  volts,  gives  a 
throw  on  the  galvanometer  of  50  divisions  when  it  is  discharged.     The  same  galva- 
nometer will  deflect  100  divisions  when  a  I  microfarad  condenser  charged  to  5  volts  is 
discharged  through  it.     \Vhat  is  the  capacity  of  the  line  ?     (Aid :  The  galvanometer 
would  deflect  2.5  divisions  if  the  line  were  charged  to  5  volts.)    Ans.  .025  microfarad. 

194.    Electrical  Units.  —  The  following  table  gives  a  review  of  the 
units  that  have  been  explained  in  this  and  the  preceding  chapters  :  — 

Ampere  =  unit  of  current. 

Milliampere  =  one-thousandth  of  an  ampere. 

Microampere  =  one-millionth  of  an  ampere. 

Volt  =  unit  of  pressure. 

Millivolt  =  one-thousandth  of  a  volt. 

Microvolt  .                             =  one-millionth  of  a  volt. 

Ohm  =  unit  of  resistance. 

Megohm  =  1,000,000  ohms. 

Coulomb  =  unit  of  quantity. 

Farad  =  unit  of  capacity. 

Microfarad  =  one-millionth  of  a  farad. 

Watt  =  unit  of  power. 

Kilowatt  =  1000  watts. 

Horse  power  =  746  watts. 

Joule  =  unit  of  work  =  one  watt  second. 

Watt  hour  =  unit  of  work  =  3600  watt  seconds. 

Kilowatt  hour  =  1000  watt  hours. 
Horse  power  hour  (H.  P.  H.)    =  746  watt  hours. 


MEASUREMENT  OF  CAPACITY  211 

The  prefixes  micro,  milli,  kilo,  and  meg  (or  mega),  which  respectively  mean  one- 
millionth,  one-thousandth,  one  thousand,  and  one  million,  may  be  applied  .to  any  of 
the  electrical  units.  For  instance,  one  kilovolt  means  one  thousand  volts,  one 
microhm  means  one-millionth  of  an  ohm,  etc. 

QUESTIONS 

1.  How  can  the  power  in  a  direct  current  circuit  be  measured  by  means  of  an 
amperemeter  and  a  voltmeter  ? 

2.  What  is  a  wattmeter? 

3.  How  can  an  electrodynamometer  be  arranged  for  use  as  a  wattmeter? 

4.  How  would  you  connect  a  wattmeter  to  measure  the  power  used  by  a  bank  of 
incandescent  lamps  in  parallel? 

5.  What  would  happen  if  the  current  coil  of  a  wattmeter  was  connected  into  a 
circuit  where  the  pressure  coil  should  be? 

6.  What  is  a  recording  wattmeter? 

7.  What  is  a  watt  hour  ? 

8.  Describe  a  Thomson  recording  wattmeter. 

9.  What  is  the  principle  of  the  retarding  device  in  the  Thomson  wattmeter? 

10.  What  is  an  ampere  hour? 

11.  \Vhy  cannot  the  difference  of  pressure  between  an  insulated  charged  body  and 
the  earth  be  measured  by  an  ordinary  voltmeter? 

12.  How  should  an  electrostatic  voltmeter  be  connected  up  to  measure  the  press- 
ure of  a  charged  body? 

13.  What  is  "specific  inductive  capacity"? 

14.  Of  the  common  materials,  which  has  the  least  inductive  capacity? 

15.  What  are  the  numerical  values  of  the  specific  inductive  capacities  of  gutta- 
percha  and  glass? 

1 6.  W7hy  is  crinkled  paper  better  than  rubber  for  insulating  telephone  cables? 

1 7.  Why  have  wires  placed  under  ground  a  greater  capacity  than  the  same  wires 
if  placed  overhead? 

1 8.  What  is  a  microfarad? 

19.  How  are  standard  condensers  made? 

20.  What  is  the  combined  capacity  of  condensers  connected  in  parallel? 

21.  What  is  the  combined  capacity  of  equal  condensers  connected  in  series? 

22.  What  is  the  combined  capacity  of  unequal  condensers  connected  in  series? 

23.  When  are  condensers  connected  in  cascade? 

24.  What  are  standard  condenser  plates  and  dielectrics  commonly  made  of? 

25.  What  is  a  ballistic  galvanometer? 

26.  What  is  the  "  throw  "  of  a  galvanometer? 

27.  If  a  ballistic   galvanometer   having   poor    insulation  were  used  in    capacity 
measurements,  what  would  happen? 

28.  Name  the  various  electrical  units  that  you  have  become  acquainted  with. 


CHAPTER    XV 


PRINCIPLES  AND    CONSTRUCTION   OF    DIRECT   CURRENT  DYNAMOS 

AND   MOTORS 

195.  Direct  Current  Dynamos.  —  A  dynamo  consists  essentially  of  a 
machine  for  transforming  mechanical  energy  into  electrical  energy,  or 
vice  versa,  through  the  intervention  of  electromagnetic  induction.     As 
already  stated  in  an  earlier  chapter,1  Faraday  discovered,  about  1830, 
that  a  conductor  cutting  lines  of  force,  when  part  of  a  closed  circuit, 
will  produce  a  current.     He  then  constructed  crude  machines  for  util- 
izing this  phenomenon,  and  he  may,  therefore,  be  fairly  considered  to 
be  the  primary  inventor  of  the  dynamo.     During  the  following  years 
many  investigators,   some   of  whose   names    are   famous,   entered    this 

fascinating  field  of  dis- 
covery ;  and  within  a 
very  few  years  they  dis- 
placed the  permanent 
magnet  for  furnishing 
the  magnetic  field  by 
the  electromagnet,  built 
up  the  armature  cores 
of  laminations,  con- 
structed commutators, 
and  used  more  than 

one  pair  of  poles.  During  the  decade  of  the  fifties,  Siemens,  Gramme, 
and  Pacinotti  appeared  with  improvements  which  developed  the  dynamo 
into  nearly  its  present  form,  but  minor  improvements  have  been  and  are 
being  continually  perfected. 

196.  Single  Coil  Dynamo.  —  If  a  coil  is  mounted  on  an  axis  or  shaft,  so 
that  it  may  be  revolved  in  a  magnetic  field  (Fig.  114),  a  condition  exists 

1  Article  132. 
212 


FIG.  114.  —  Coil  arranged  to  be  rotated  in  Magnetic  Field. 


DIRECT  CURRENT  DYNAMOS  AND   MOTORS 


213 


FIG.  115.— Ends  of  Coil  con- 
nected to  Sliding  Rings, 
AA,  with  Brushes,  BBt 
making  Connection  with 
External  Circuit. 


which  is  described  in  the  last  paragraph  of  Article  136,  and  an  alternat- 
ing current  is  produced  in  the  coil  when  it  is  revolved. 

If,  instead  of  being  short-circuited  on  itself, 
the  coil  is  connected  to  an  external  circuit  by 
means  of  such  sliding  contacts  as  are  shown 
in  Figure  115,  the  alternating*  current  may  be 
led  off  to  be  used  for  any  desired  purpose. 
The  rings  A  A,  to  which  the  ends  of  the  coils 
are  attached,  in  this  case  are  called  Collecting 
Rings  or  Collectors,  and  the  parts  BB,  which 
bear  on  the  collectors,  are  called  Brushes.  In 
an  actual  machine  made  up  for  the  purpose 
of  generating  electricity  by  a  coil  revolving  in 
a  magnetic  field,  the  revolving  part  is  called 
an  Armature.  Telephone  Magnetos,  which 
are  used  for  ringing  telephone  call  bells,  con- 
sist of  a  coil  of  wire  wound  on  an  iron  core,  which  is  revolved  in  the 
magnetic  field  between  the  poles  of  a  permanent  horseshoe  magnet 

(Fig.  116).  An  enlarged  cross 
section  of  the  armature  is  shown 
alongside  of  the  complete  machine 
in  Figure  116.  Such  machines 
produce  alternating  currents. 

197.  Commutators.  —  It  is  pos- 
sible to  arrange  the  collector  which 
is  attached  to  a  coil  that  is  revolved 
in  a  magnetic  field  in  the  manner 
shown  in  Figure  117,  With  this 
arrangement,  the  collector  seg- 
ments connect  each  brush  first 
with  one  end  of  the  coil  and  then 
with  the  other  end  as  the  coil  revolves.  If  the  brushes  are  properly  Set 
(that  is,  if  they  bear  on  the  collector  at  proper  points),  this  arrangement 
causes  the  current  to  flow  continuously  in  one  direction  in  the  external 
circuit,  though  in  the  coil  itself,  the  direction  of  current  flow  reverses  with 
each  half  revolution  as  before.  Such  an  arrangement  of  the  collector  is 


ARMATURE 


FIG.  116.  —  Magneto  Generator  of  Alternat- 
ing Currents. 


214 


ELECTRICITY   AND    MAGNETISM 


called  a  Commutator,  and  the  current  in  the  outside  circuit  is  said  to  be 
Commutated  or  Rectified.  The  brushes  must  be  set  on  opposite  sides  of 
the  commutator  and  between  the  pole  tips,  otherwise  little  or  no  current 
will  be  sent  into  the  external  wire  and  much  sparking  may  result.  Fig- 
ure 118  shows  one  of  the  early  dynamos  with  a  single  coil  armature  and 


FIG.  117.  —  Collector  with  Two  Segments. 


FiG.  118.  — Old  Style  Small  Dynamo. 


commutator  of  two  segments.  This  machine  looks  quite  like  the  mag- 
neto shown  in  Figure  116,  but  the  collector  is  different,  and  the 
magnetic  field  is  set  up  by  an  electromagnet  instead  of  a  permanent 
magnet. 

198.  Current  Wave  in  a  One-coil  Armature.  —  An  armature  with  one 
coil  and  a  two  part  commutator  furnishes  a  current  consisting  of  a  series 
of  waves,  or  pulsations,  which  may  be  represented  by  Figure  119.  This  is 

easily  understood  after  a  little  considera- 
tion. When  the  coil  stands  up  and  down 
between  the  pole  pieces  as  in  Figure  114 
and  the  full  lines  in  Figure  120,  it  is  in 
such  a  position  that  when  it  is  revolved 
a  small  amount,  the  conductors  move 
practically  parallel  to  the  lines  of  force 

and  no  lines  are  cut.  When  the  coil  is  in  continuous  revolution,  no 
pressure  is  induced  at  the  instant  that  it  is  in  the  positions  shown  in 
Figure  114  and  the  full  lines  in  Figure  120,  which  correspond  with  the 
points  A,  C,  and  A'  in  Figure  119. 


A  c  A' 

FIG.  119. —  Diagram  of  Current  Pul- 
sations. 


DIRECT   CURRENT   DYNAMOS  AND   MOTORS  21$ 

When  the  coil  stands  as  shown  by  the  dotted  lines  in  Figure  120,  it  is 
in  such  a  position  that  the  conductors  cut  squarely  across  the  lines  of 
force  as  they  move,  and  the  largest  possible  number  of  lines  of  force  are 


FlG.  120.  —  Coil  arranged  to  be  rotated  in  Magnetic  Field. 


cut  for  a  given  amount  of  motion.  The  dotted  position  of  the  coil 
shown  in  the  figure  and  the  position  180°  therefrom  correspond  with  the 
points  B  and  D  in  Figure  119. 

PROBLEMS. 

A.  Suppose  we  have  a  single  coil  armature  of  20  turns  in  a  magnetic  field  of  such 
strength  that  each  conductor  cuts  1,000,000  lines  in  each  half-revolution.     If  the  ma- 
chine runs  at  1500  revolutions  per  minute,  what  is  the  average  pressure  developed  ? 
(Aid :  Each  conductor  cuts  2,000,000  lines  in  a  revolution,  but  each  turn  has  two  cut- 
ting conductors.      Therefore  each  turn  will  cut  4,000,000  lines  in   each  revolution. 
Twenty  turns  give  an  effect  twenty  times  as  great,  which  is  equivalent  to  cutting 
80,000,000  lines  'per  revolution  by  one  turn.     The  armature  makes  1500  -=-  60  =  25 
revolutions  per  second,  hence  the  coil  gives  the  effect  of  cutting  25  x  80,000,000  lines 
per  second.     This  is  equal  to  2,000,000,000  lines  cut  per  second.     But  to  create  a 
pressure  of  one  volt  requires  that  100,000,000  lines  be  cut  per  second.1     Therefore 
the  machine  will  develop  2^0^0000^  _  2O  Volts.) 

B.  Suppose  we  have  a  single  coil  armature  of  50  turns  in  a  field  of  such  strength 
that  each  conductor  cuts  2,000,000  lines  of  force  per  each  half-revolution.     If  the 
speed  is  1200  revolutions  per  minute,  what  is  the  average  pressure  developed  ?    Ans. 
80  volts. 

199.  Armatures  having  More  than  One  Coil ;  Gramme  Ring.  —  Direct 
current  dynamos  having  armatures  with  one  coil  are  not  satisfactory  for 
general  use  for  two  reasons  :  — 

i.  The  wavy  character  of  the  current  is  a  disadvantage  for  some  pur- 
poses. 

1  Article  134. 


216 


ELECTRICITY   AND   MAGNETISM 


FIG.  121.  —  Gramme  Armature. 


2.  The  commutation  of  large  currents  at  the  full  pressure  which  is 
required  for  most  commercial  uses  is  not  practical. 

To  overcome  these  difficulties  coils  must  be  uniformly  distributed 
over  the  surface  of  the  armature,  and  the  windings  must  be  connected 
at  equal  intervals  to  commutator  segments.  The  first  armature  of  this 

kind  that  was  put  into  commercial  service 
was  invented  in  1870  by  a  Frenchman 
named  Gramme.  The  core  of  Gramme's 
armature  consisted  of  a  ring  made  of  iron 
wire.  This  ring  had  a  winding  made  of 
insulated  copper  wire  wound  uniformly  over 
its  surface,  and  at  equal  intervals  the  wind- 
ings were  electrically  connected  to  com- 
mutator segments.  The  arrangement  is 
shown  in  Figure  121.  When  this  armature 

is  placed  in  a  magnetic  field,  the  lines  of  force  pass  through  the  iron 
armature  core  from  one  pole  to  another  in  the  way  that  is  illustrated  in 
Figure  122,  so  that  the  revolution  of  the  ring  causes  the  outer  conductors 
to  cut  lines  of  force,  but  the  inner  conductors  are  entirely  shielded. 

When  the  armature  is  revolved,  the  wires  of  the  armature  wind- 
ing which  are  under  one 
pole  piece  cut  lines  of 
force  in  one  direction,  and 
those  under  the  other  pole 
piece  cut  lines  in  the  oppo- 
site direction.  The  effect 
of  the  opposing  electric 
pressures  which  are  thus 
set  up  in  the  windings  of 
the  armatures,  is  to  cause 

a  point  at  one  side  of  the  armature  to  come  to  a  high  electrical  pressure 
and  a  point  on  the  opposite  side  to  come  to  a  low  electrical  pressure. 

If  brushes  bear  on  the  commutator  at  these  points  (A  and  B  in  Fig. 
123),  a  current  flows  in  the  external  circuit  from  the  high  to  the  low 
pressure  side  of  the  armature,  that  is,  from  A  to  B.  The  path  of  the 
current  through  the  armature  itself  is  from  B  to  A,  through  the  two 


FIG.  122.  —  Illustration  of  Way  in  which  Magnetism 
passes  through  Gramme  Armature  Core. 


DIRECT   CURRENT   DYNAMOS  AND   MOTORS 


217 


halves   of  the   armature  in  parallel.      This  is  plainly  shown  by  the 
figure. 

Since  the  number  of  conductors  under  the  pole  pieces  is  practically 
the  same  for  every  position  of  the  armature  during  the  revolution,  the 
armature  produces  a  practically  uniform  pressure  when  it  is  con- 
tinuously revolved  at 
a  uniform  rate,  as 
when  it  is  driven  by 
a  steam  engine. 

As  a  rule,  com- 
mercial Gramme  or 
Ring  Armatures  are 
not  wound  with  a 
continuous  wire,  but 
the  divisions  of  the 

armature      windings, 

FIG.  123.  —  Gramme  Armature,  showing  the  way  in  which 
the     ends    Of     which  Current  flows  through  it. 

are  connected  to  ad- 
jacent Commutator  Segments  or  Bars,  are  wound  as  separate  coils. 
This  makes  it  possible  to  insulate  the  different  parts  of  the  winding 
more  effectively  from  each  other,  and  thus  prevent  the  current  from 
jumping  by  a  short  path,  or  Short-circuiting,  directly  from  one  coil  to 
another,  instead  of  following  all  the  way  around  the  coils.  The  separate 
coils  are  connected  to  the  commutator  segments,  and  to  each  other,  in 
such  a  way  that  the  winding  is  in  effect  the  same  as  though  made  with 
a  continuous  wire  connected  at  intervals  to  the  commutator  segments. 


PROBLEMS 

A,  A  Gramme  armature  has  50  coils  of  5  turns  each.  Two  million  lines  of  force 
pass  through  this  armature.  If  the  speed  is  600  revolutions  per  minute  (equal  to  10 
revolutions  per  second),  what  pressure  is  developed?  (Aid:  In  a  Gramme  armature 
there  is  one  cutting  conductor  per  turn.  Each  conductor  cuts  the  two  million  lines 
twice  in  a  revolution,  and  the  total  number  of  lines  cut  per  revolution  by  all  the  con- 
ductors is  therefore  1,000,000,000.  If  all  the  coils  worked  in  series,  the  pressure 
would  be  100  volts,  but  since  this  pressure  is  divided  into  two  paths  in  parallel, 
the  pressure  between  the  brushes  is  50  volts.)  Ans.  50  volts. 


218 


ELECTRICITY  AND   MAGNETISM 


B.  A  Gramme  armature  has  60  coils  of  two  turns  each.    Five  million  lines  of  force 
pass  through  the  armature.    If  the  speed  is  900  revolutions  per  minute,  what  pressure 
is  set  up?     Ans.  90  volts. 

C.  A  Gramme  armature  as  in  Example  A  revolves  1200  revolutions  per  minute.    If  it 
is  to  set  up  100  volts,  how  many  lines  of  force  must  there  be  in  the  field  ?   Ans.  2,000,000. 

D.  Suppose  the  armature  of  Example  B  was  to  develop  45  volts,  how  many  turns 
would  the  armature  need  to  carry  if  the  speed  and  field  were  not  changed?    Ans.  60. 

200.  Drum  Armatures.  —  The  armature  core  may  be  an  iron  cylinder 
or  drum,  made  out  of  disks  of  sheet  iron  laid  together  (Fig.  124),  in- 
stead of  an  iron  ring.  In  this  case  the  winding  seems  more  complicated, 
but  its  general  plan  is  similar  to  that  of  the  ring  armature.  The  winding 
consists  of  a  number  of  coils  wound  uniformly  over  the  surface  of  the 


FIG.  124. —  Drum  Armature  Core,  showing  its  Laminated  Character. 

drum,  which  are  connected  together  in  such  a  way  that  the  winding  is 
electrically  the  same  as  though  it  had  been  made  with  a  single  long  wire. 
The  coils  are  connected  to  the  commutator  bars  exactly  as  in  the  ring 
armature,  and  their  effect  in  producing  electrical  pressure  when  the 
armature  is  revolved  is  just  the  same  as  has  already  been  explained  in 

the  case  of  the  ring  armature.  Arma- 
tures with  drum-shaped  cores  are  called 
Siemens  or  Drum  Armatures. 

A  Siemens  armature  with  four  coils  is 
shown  in  Figure  125,  from  which  may 
be  seen  the  way  in  which  the  wires  are 
wound  on  the  core  and  connected  to  the 
commutator.  It  is  seen  from  this  figure 
that  both  sides  of  the  coil  cut  lines  of 
force  in  a  drum  armature,  but  as  they 

are  under  opposite  poles  the  current  in  one  side  tends  to  go  toward 
the  back  and  the  other  toward  the  front  of  the  armature.  It  is,  therefore, 


FiG.  125.  —  Drum  Armature  with  Four 
Coils  and  Commutator  of  Four 
Segments. 


DIRECT  CURRENT   DYNAMOS  AND   MOTORS  2IQ 

evident  that  the  pressures  in  the  conductors  on  the  two  sides  of  an 
armature  add  together  to  cause  the  current  to  circulate  properly  through 


FIG.  126.  —  Drum  Armature  Core  with  Commutator  of  Sixteen  Segments,  showing  One 
Coil  wound  on  the  Core. 

the  coils.  Arrows  indicating  the  relative  directions  are  shown  in  the 
figure.  Figure  126  shows  one  coil  wound  upon  an  armature  core 
which  is  intended  for  sixteen  coils.  The  armatures  of  commercial 
dynamos  usually  have  from  thirty  to  one  hundred  coils. 

PROBLEMS 

A.  A  Siemens  armature  has  25  coils  of  five  turns  each.     Two  million  lines  of  force 
pass  through  the  armature.     If  the  speed  is  600  revolutions  per  minute,  what  will  be 
the  pressure  developed?     (Aid:  In  a  drum  armature  each  turn  has  two  cutting  con- 
ductors, so  that  the  total  number  will  be  the  same  as  in  Example  A,  Article  199.) 
Ans.  50  volts. 

B.  A  Siemens  armature  is  to  develop  100  volts  in  a  field  having  4,000,000  lines 
of  force.     How  many  turns  (two  conductors  to  the  turn)  must  the  armature  bear  if 
the  speed  is  to  be  1500  revolutions  per  minute?     Ans.  50  turns. 

201.  Core  Laminations;  Eddy  Currents.  —  It  has  already  been  said 
that  the  early  Gramme  armature  Cores  were  made  out  of  iron  wire 
coiled  up  to  form  a  ring.  In  modern  machines  the  cores  for  both 
Gramme  and  Siemens  armatures  are  built  up  of  disks,  which  are 
punched  out  of  sheet  iron  (Fig.  124).  These  disks  are  usually  insulated 
from  each  other  by  thin  tissue  paper,  or  by  thin  coverings  of  varnish  or 
non-conducting  oxide.  The  object  of  dividing  the  cores  into  disks,  or 
Laminating  them,  and  of  insulating  the  disks  from  each  other,  is  to 
prevent  currents  from  being  set  up  in  the  core  itself  when  it  is  revolved 
in  the  magnetic  field.  The  rule  that  electric  pressures  are  set  up  when 
a  conductor  cuts  lines  of  force  applies  equally  as  much  to  the  core  of 


220  ELECTRICITY   AND   MAGNETISM 

the  armature  as  to  the  windings.  Currents  tend  to  flow  in  armature 
cores  from  one  end  to  the  other  near  the  surface  under  one  magnet  pole, 
and  to  return  under  the  opposite  pole.  By  properly  laminating  the  cores, 
these  currents  are  nearly  all  prevented,  while  the  passage  of  lines  of 
force  through  the  iron,  from  one  side  of  the  core  to  the  other,  is  not 
interfered  with. 

The  great  objection  to  permitting  currents  to  circulate  in  the  armature 
core  is  the  fact  that  it  takes  power  to  keep  them  circulating,  and  all 
this  power  is  converted  into  heat  in  the  armature  core,  and  is  wasted. 
The  heating  of  the  core  has  another  disadvantage,  since  a  high  tempera- 
ture is  likely  to  injure  the  cotton  and  shellac  insulation  which  is  used 
between  the  coils  themselves,  and  between  the  coils  and  core.  Even 
with  the  best  of  Lamination  a  certain  amount  of  power  is  lost,  and 
heating  is  caused,  by  currents  circulating  in  the  core  disks.  These 
currents  are  ordinarily  called  Eddy  Currents,  because  they  eddy  uselessly 
through  the  core,  or  Foucault  Currents,  after  the  name  of  a  scientist  who 
made  some  investigations  many  years  ago  relating  to  the  generation 
of  currents  in  masses  of  metal. 

202.  Hysteresis  Loss.  —  There  is  an  additional  cause  of  lost  power 
and  heating  in  the  cores  of  armatures  which  cannot  be  reduced  by 
lamination.  This  seems  to  be  due  to  a  sort  of  friction  between  the 
molecules  as  they  are  caused  to  turn  over  by  the  attraction  of  the 
magnetic  field  while  the  armature  revolves.  Every  time  the  molecules 
are  caused  to  turn  around  under  the  influence  of  a  magnetic  field,  a 
certain  amount  of  power  is  used,  which  is  converted  into  heat ;  con- 
sequently, for  every  revolution  of  the  armature,  a  certain  amount  of 
power  is  used,  and  converted  into  heat.  This  effect  is  another  result  of 
the  phenomenon  of  magnetism  which  is  called  Hysteresis,  and  which  is 
described  in  Article  128. 

The  amount  of  power  wasted  and  heat  produced  in  a  core  on  account 
of  hysteresis  depends  upon  the  amount  of  iron  in  the  core,  the  num- 
ber of  revolutions  made  by  it  in  a  minute,  the  density  of  magnetism  in 
the  iron,  and  the  quality  of  the  iron.  It  may  be  said  that,  in  general,  the 
softer  the  iron  the  less  is  the  loss  due  to  hysteresis ;  consequently,  the 
iron  used  in  armature  cores  is  very  soft  wrought  iron  or  steel  which  has 
been  carefully  annealed. 


DIRECT  CURRENT   DYNAMOS  AND   MOTORS 


221 


~    12/f    • 

I       p  '   1 

g 

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£ 

1 

1 

1    PI  1 

JPT. 

203.  Field  Magnets.  —  The  magnetic  field  in  which  the  armature 
revolves  is  ordinarily  produced  by  a  great  electromagnet.  The  frame 
of  this  electromagnet  is  so  arranged  that  it  can 
hold  the  windings  required  to  set  up  the  lines 
of  force ;  and  in  order  that  the  lines  may  be 
caused  to  pass  through  the  armature,  the  poles 
are  arranged  to  embrace  the  armature.  These 
expanded  poles  are  called  Pole-pieces  (PP  in 

Figs.  127  and  128).  and  the  whole  of  the  mag- 

5  r  .         „    V  FIG.  127.  — Diagram  of  Dy- 

net  frame  is  called  the  Field  of  the  machine.  namo  Frame. 

The  parts  of  the  field  upon  which  the  windings 

are  placed  are  often  called  the  field  Cores  (mm,  in  Figs.  127  and  128). 
The  portion  of  the  magnet  that  connects  the  cores  is 
called  the  Yoke  (y  in  Figs.  127  and  128). 

204.  Air  Space.  —  It  is  always  necessary  to  allow 
a  certain  amount  of  space  between  the  pole-pieces 
and  the  surface  of  the  armature,  and  in  addition  a 
certain  amount  of  space  is  occupied  by  the  armature 
windings,  so  that  a  considerable  depth  of  non-mag- 
netic material  exists  between  the  iron  of  the  pole- 
pieces  and  the  jron  of  the  armature  core.  This  space 
is  usually  called  the  Air  Space  or  Gap  (G,  Figs.  127 
and  128). 

205.  Reluctance  of  Magnetic  Circuit.  — The  number  of  ampere  turns, 
which  are  required  to  give  the  magnetomotive  force  that  is  needed  to 
set  up  the  lines  of  force  necessary  to  induce  a  given  electrical  pressure 
in  the  armature  windings,  depends  upon  the  reluctance  of  the  armature 
core,  of  the  air  gap,  and  of  the  magnet  frame.  Since  there  is  no 
insulator  of  magnetism,  some  of  the  lines  of  force  which  are  set  up  in 
the  field  will  leak  around  the  armature  instead  of  passing  through  it, 
and  the  cross  section  of  iron  in  the  path  of  the  lines  of  force  -through 
the  field  must  be  sufficiently  large  to  hold  these  leakage  lines  as  well 
as  the  useful  ones  which  pass  through  the  armature.  It  is  the  Leakage, 
or  stray  lines  of  force,  which  magnetize  watches  when  they  are  carried 
near  a  dynamo. 

In  order  that  the  proportion  of  the  total  number  of  lines  of  force  that 


FIG.  128.  —  Dia- 
gram of  Dynamo 
Frame. 


222  ELECTRICITY   AND    MAGNETISM 

leak  around  the  armature  shall  be  as  small  as  possible,  the  reluctance  of 
the  air  gap,  which  is  always  a  considerable  part  of  the  total  reluctance 
in  the  magnetic  circuit,  must  be  made  as  small  as  possible. 

PROBLEMS 

A.  The  reluctance  of  the  magnetic  circuit  of  a  dynamo  is  .002.     How  many  am- 
pere turns  will  be  required  to  create  a  field  of  4,000,000  lines?1 

(  Aid  :  Apply  formula  N  =  l'2^  nc  or  nc  =  -=^-.  J      Ans.  6360  (approx.). 

B.  The  reluctances  of  the  parts  of  the  magnetic  circuit  of  a  certain  dynamo  are 
calculated  to  be  as  follows:     Yoke,  .oooi;   field  cores,  .0002;   pole  pieces,  .0002; 
armature,  .00075;   anc^  a^r  spaces,  .00155.     How  many  ampere  turns  are  required  to 
set  up  a  field  of  2,000,000  lines  of   force?      (Aid:    Add  the  reluctances  together, 
since  they  are  in  series.)     Ans.  4460  (approx.). 

C.  The  magnetic  circuit  of  a  dynamo  has  a  reluctance  of  .004.     The  armature  of 
the  dynamo  requires  2,000,000  lines  of  force  to  set  up  its  normal  pressure.     If  the 
magnet  coils  are  to  have  2  amperes  passed  through  them,  how  many  turns  must  they 
have?     Ans.  3180  (approx.). 

D.  If  the  pressure  supplied  by  the  dynamo  to  the  field  coils  of  Example  C  is  100 
volts,  what  must  be  the  resistance  of  the  coils?     Ans.  50  ohms. 

206.  Toothed  Armatures.  —  It  is  also  of  advantage  to  make  the  air 
gap  reluctance,  and  therefore  the  total  reluctance  of  the  magnetic  cir- 
cuit, as  small  as  possible,  because  the  number  of  ampere  turns  which 


FlG.  129.  —  Complete  Armature  with  a  "  Toothed  "  or  "  Slotted  "  Core. 

are  required  to  set  up  the  field  magnetism  are  thereby  reduced, 
and  the  expense  of  building  the  machine  is  consequently  decreased. 
For  this  purpose,  the  armature  core  is  often  made  toothed,  and  the 

1  Magnetic  leakage  is  not  considered  in  these  problems.  2  Article  131. 


DIRECT  CURRENT  DYNAMOS   AND   MOTORS  223 

windings  are  placed  in  the  slots  or  grooves  between  the  teeth  (Fig. 
129). 

It  is  sometimes  thought  that  placing  the  armature  conductors  in 
grooves  between  teeth  in  the  core  permits  some  of  the  lines  of  force  to 
pass  through  the  core  in  such  a  way  that  they  are  not  cut  by  the  con- 
ductors as  the  armature  revolves.  This  is  a  mistake,  however,  and  an 
armature  with  the  conductors  wound  in  slots  gives  exactly  the  same 
electrical  pressure  when  revolved  in  a  magnetic  field  as  is  given  by  an 
armature  with  the  same  number  of  conductors  wound  on  the  surface  of 
its  core  when  it  is  revolved  at  the  same  speed  in  a  field  of  the  same 
strength. 

QUESTIONS 

1.  What  had  Faraday  to  do  with  development  of  the  dynamo? 

2.  Give  a  brief  history  of  the  development  of  the  dynamo. 

3.  What  is  the  action  of  a  single  coil  dynamo? 

4.  Is  the  slider  arrangement  described  in  Art.  134  a  dynamo  in  principle? 

5.  What  are  collecting  rings? 

6.  What  are  dynamo  brushes? 

7.  What  is  a  dynamo  armature? 

8.  What  is  a  magneto? 

9.  What  is  a  commutator? 

10.  How  does  a  commutator  act? 

11.  What  is  a  rectified  current? 

12.  Where  must  the  brushes  be  set  on  the  commutator? 

13.  What  is  the  form  of  the  rectified  current  delivered  from  a  one-coil  armature? 

14.  Why  are  not  one-coil  armatures  used  commercially? 

15.  Who  invented  the  first  commercially  used  many-coiled  armature ?     When? 

1 6.  How  is  a  Gramme  armature  wound? 

17.  How  does  the  current  flow  through  a  Gramme  armature? 

1 8.  Why  are  there  two  parallel  paths  for  the  current  in  a  Gramme  armature? 

19.  Why  does  a  many-coiled  armature  produce  a  practically  constant  pressure? 

20.  What  is  a  commutator  bar? 

21.  What  is  a  Siemens  armature? 

22.  How  is  a  Siemens  armature  wound? 

23.  Compare  drum  and  ring  windings. 

24.  Why  do  the  pressures  set  up  on  the  two  sides  of  a  drum  armature  coil  add 
together? 

25.  What  is  the  armature  core?     Why  is  it  made  of  iron? 

26.  How  are  armature  cores  made? 


224  ELECTRICITY  AND   MAGNETISM 

27.  Why  are  armature  cores  laminated? 

28.  What  are  Foucault  or  eddy  currents? 

29.  Why  are  Foucault  currents  disadvantageous  in  the  core  of  an  armature? 

30.  What  effect  has  hysteresis  in  an  armature  core? 

31.  What  does  the  hysteresis  loss  depend  upon? 

32.  How  are  dynamo  field  magnets  made? 

33.  What  is  the  field  of  a  dynamo? 

34.  What  are  the  pole-pieces?     The  yoke?     The  field  core?     The  air  gap? 

35.  Should  the  reluctance  of  a  dynamo  field  be  great  or  small? 

36.  What  effect  has  the  quality  of  iron  and  length  of  air  gap  on  the  field  reluc- 
tance? 

37.  Why  is  a  watch  likely  to  be  magnetized  when  brought  near  a  dynamo? 

38.  What  is  magnetic  leakage? 

39.  What  effect  has  the  length  of  the  air  gap  upon  magnetic  leakage  ? 

40.  What  is  a  toothed  armature? 

41.  What  advantages  have  toothed  armatures? 

42.  Will  a  toothed  armature  give  any  less  pressure  than  a  smooth  one? 

207.  Electric  Motors.  —  We  have  seen  that  the  operation  of  dynamos 
is  a  direct  application  of  Faraday's  discovery,  that  an  electrical  pressure 
is  generated  in  a  conductor  when  it  is  moved  in  a  magnetic  field. 

Electric  motors  work  on  the  principle  that  a  conductor  carrying  a 
current  tends  to  move  when  placed  in  a  magnetic  field,  on  account  of  the 
mutual  action  of  the  lines  of  force  of  the  field  and  of  the  current. 
The  reasons  for  these  actions  we  do  not  know,  but  we  know  of  their 
existence  as  the  result  of  experiment,  and  are  able  to  apply  their  results 
to  practical  use. 

These  two  principles  are  practically  the  reverse  of  each  other,  and 
the  action  of  generators  and  motors  is,  therefore,  a  Reversible  one.  That 
is,  a  machine  which  is  designed  to  be  used  as  a  dynamo  to  generate 
electric  currents  when  driven  by  mechanical  power,  may  usually  be 
used  equally  well  to  generate  mechanical  power  when  driven  as  a  motor 
by  electric  currents.  It  is  a  fact  that  the  machines  that  are  the  best 
generators  also  usually  make  the  best  motors ;  and  manufacturers  sell 
their  standard  direct  current  dynamos,  to  be  used  either  as  generators 
or  motors.  We  shall,  therefore,  treat  them  as  entirely  similar  in  con- 
struction. It  is  only  when  the  machines  are  built  to  be  used  for  some 
special  purpose  that  they  cannot  be  conveniently  interchanged  in  their 
action. 


DIRECT  CURRENT  DYNAMOS  AND   MOTORS  225 

When  the  motor  armature  is  caused  to  revolve  by  the  magnetic  attrac- 
tions, its  conductors  cut  the  lines  of  force  of  the  field,  and  an  electric 
pressure  is,  therefore,  set  up  in  them,  just  as  has  been  described  in  the 
early  articles  of  this  chapter.  The  direction  of  this  is  opposite  to  that 
of  the  external  source  which  sends  the  current  through  the  armature. 
The  electric  pressure  which  is  thus  set  up  in  the  armature  conductors  of 
the  motor  is  called  a  counter  electric  pressure,  or  counter  electromotive 
force. 

The  work  which  is  done  by  the  motor  is  dependent  upon  this  counter 
electric  presstire,  and  a  useful  electric  motor  which  does  not  produce  a 
counter  electric  pressure,  is  as  impossible  of  existence  as  is  a  perpetual 
motion  machine.  Seekers  after  either  are  looking  for  the  impossible. 

When  a  dynamo  armature  is  revolved  in  a  magnetic  field  so  as  to  pro- 
duce a  current,  the  lines  of  force  belonging  to  the  current  are  attracted 
by  the  lines  of  force  belonging  to  the  field.  This  attraction  tends  to 
stop  the  motion,  so  that  power  has  to  be  exerted  to  keep  the  armature 
moving.  The  greater  the  current  the  greater  must  be  the  pull  or  Torque 
given  to  the  dynamo  pulley  to  make  it  rotate.  Likewise,  in  a  motor, 
the  current  flowing  in  the  armature  must  be  sufficient  to  give  the  neces- 
sary pull  to  beep  the  armature  going,  whatever  the  load  upon  the  motor 
pulley.  By  Ohm's  Law,  the  amount  of  current  that  will  flow  through 
the  resistance  of  the  armature  will  be  proportional  to  the  pressure.  In 
a  motor,  the  pressure  sending  currents  through  the  armature  windings 
is  the  difference  between  the  pressure  applied  to  the  armature  and  the 
counter  pressure.  Evidently,  then,  when  a  load  is  put  on  the  pulley, 
the  motor  armature  must  either  slacken  a  little  in  speed,  or  the  fields 
must  be  weakened,  to  so  reduce  the  counter  pressure  that  sufficient  cur- 
rent will  flow  through  the  armature  windings  and  give  the  proper  torque. 

This  is  a  general  rule  for  dynamos  :  The  greater  the  mechanical  work 
the  greater  must  be  the  electric  work,  or  vice  versa. 

208.  Machine  Efficiency. — The  electrical  power  delivered  by  a 
dynamo  to  the  circuit  with  which  it  is  connected  is  always  less  than  the 
mechanical  power  used  in  driving  the  machine.  The  difference  is 
absorbed  in  the  machine  itself,  and  is  transformed  into  heat  which 
warms  the  dynamo,  through  the  effects  of  friction,  hysteresis,  eddy  cur- 
rents, and  the  C^R  loss  in  the  dynamo  windings. 
Q 


226 


ELECTRICITY   AND    MAGNETISM 


If  this  difference  is  great,  —  that  is,  if  the  internal  Losses  are  great, — 
the  dynamo  may  not  be  a  satisfactory  one,  and  we  say  its  Efficiency  is 
low.  The  actual  value  of  the  efficiency  is  found  through  dividing  the 
number  of  watts  delivered  to  the  circuit  by  the  dynamo,  by  the  num- 
ber of  watts  representing  the  mechanical  power  used  in  driving  the 
machine.  In  other  words,  the  efficiency  is  equal  to  the  ratio  of  the  power 
taken  out  of  a  machine  to  the  power  put  in.  This  definition  applies  to 
all  classes  of  machinery. 

When  a  machine  is  caused  to  operate  as  a  motor  by  furnishing  current 
to  it  from  an  external  source,  the  same  losses  exist,  so  that  the  amount 
of  electrical  power  which  must  be  furnished  to  it  is  greater  than  the 
mechanical  power  which  is  taken  from  its  pulley. 


PROBLEMS 

A.  If  25  horse  power  are  used  in  driving  a  dynamo  of  15  kilowatts  capacity  when 
it  is  furnishing  its  full  capacity  to  the  external  circuit,  what  is  its  full  load  efficiency? 
(Aid:  Reduce  all  power  to  watts.)  Ans.  80.4%  (approx.). 

J3.  A  motor  requires  10  kilowatts  to  enable  it  to  supply  its  full  capacity  of  10 
horse  power  to  its  pulley.  What  is  its  full  load  efficiency?  Ans.  74.6  per  cent. 

C.  The  motor  of  Example  B  will  have  essentially  the  same  efficiency  when  driven 
as  a  dynamo.  -If  it  gives  to  the  external  circuit  7460  watts  when  running  as  a 

dynamo  at  full  load,  how  many  horse  power 
•will  be  required  to  drive  it?  Ans.  13.4 
H.  P.  (approx.). 

209.  Series,  Shunt,  and  Compound 
Wound  Dynamos  and  Fields.  —  The 
exciting  current  for  the  fields  of  a 
dynamo  is  almost  universally  gener- 
ated by  the  machine  itself,  the  press- 
ure at  starting  being  obtained  by  the 
slight  residual  magnetism  that  re- 
mains in  the  magnets.  In  order  to 
distinguish  the  several  methods  of 
winding  the  fields,  dynamos  may  be 
FIG.  130. -Diagram  of  Series-wound  Dy-  divided  into  three  classes.  These 
namo.  are  :  — 


DIRECT  CURRENT   DYNAMOS   AND   MOTORS 


227 


1.  Series-wound  (Fig.  130),  in  which  the  field  winding  is  connected 
in  series  with  the  external  circuit,  and  all  the  current  generated  by  the 
dynamo   passes    through   a  thick 

wire  which  is  wound  a  compara- 
tively few  times  around  the  field 
cores. 

2.  Shunt- wound  (Fig.  131),  in 
which  a  field  winding  of  high  re- 
sistance is  connected  in  parallel, 
or  as  a  shunt,  to  the  external  cir- 
cuit, and  only  a  portion  of  the 
current  generated  by  the   dyna- 
mo passes  around  the  field  cores 
through   a   great   many  turns  of 
fine  wire. 

3.  Compound- wound  (Fig.  132), 
which   is   a   combination   of    the 
first  two,  so   that   the   fields  are 

magnetized  in  the  same  direction  by  both  a  shunt  and  a  series  winding. 
If  three  dynamos  of  the  same  size  and  shape  have  fields  wound  in  the 

three  different  ways,  the  number  of  ampere  turns  in  the  magnetizing 

coils  must  be  the  same  in  each. 
Since  the  series  winding  carries 
a  large  current,  the  number  of 
times  the  current  must  pass 
around  the  magnet  core  to 
make  a  given  number  of  am- 
pere turns  is  comparatively 
small,  and  the  winding  has 
comparatively  few  turns.  The 
shunt  winding  carries  a  com- 
paratively small  current,  and 
this  current  must,  therefore, 
pass  many  times  around  the 


FIG.  131.  —  Diagram  of  Shunt-wound   Dy- 
namo. 


FiG.   132.  —  Diagram  of  Compound-wound 
Dynamo. 


core  in  order  that  it  may  have 
the  same  magnetizing  effect  as 


228 


ELECTRICITY  AND   MAGNETISM 


FlG.  133.  —  Connection  of  Series  Arc  Light  Cir- 
cuit with  Series-wound  Dynamo. 


the  large  current  passing  a  few  times  around  the  core.  In  the  com- 
pound winding,  the  number  of  series  turns  and  of  shunt  turns  must  be 
so  proportioned  that  the  number  of  ampere  turns  made  up  by  both 
together  shall  be  approximately  the  same  as  in  the  other  cases. 

210.    Characteristics  of  Field  Windings.  —  The  purpose  for  which  a 
dynamo  is  to  be  used  almost  always  fixes  the  style  of  its  field  windings. 

Series-wound  dynamos  are  or- 
dinarily used  for  furnishing  a 
current  of  constant  strength  to 
arc  lamps  which  are  connected 
in  series  (Fig.  133).  Series 
windings  are  also  used  on  the 
fields  of  street  railway  motors. 
Shunt  or  compound  wound 
dynamos  are  used  for  furnish- 
ing the  current  to  incandes- 
cent lamps  or  electric  motors 
which  are  all  connected  in  par- 
allel (Fig.  134),  between  wires  which  are  kept  at  a  constant  difference 
of  pressure ;  and  shunt-wound  motors  are  commonly  used  to  furnish 
power  for  stationary  purposes. 
Compound  dynamos  have 
quite  an  advantage  for  furnish- 
ing current  to  be  used  by  elec- 
tric motors,  that  is,  for  power 
distribution,  because  they  au- 
tomatically keep  the  pressure 
constant  through  the  combined 
action  of  the  shunt  and  series 
field  windings.  The  pressure 
supplied  by  shunt  dynamos  de- 
creases to  a  certain  degree  as 

the     current    furnished    by    the    FlG*  I34- —  Connection  of  Lamps  arranged  in 

Parallel  with  Shunt-wound  Dynamo. 

armature  increases,  on  account 

of  the  resistance  of  the  armature,  and  because  the  magnetism  set  up  by 

the  current  in  the  armature  coils  interferes  with  the  field  magnetism. 


DIRECT  CURRENT  DYNAMOS   AND   MOTOR 


229 


The  magnetizing  power  of  a  series  winding,  of  course,  increases  with 
the  current  which  is  furnished  by  the  machine,  and  the  natural  fall  of 
pressure  in  a  shunt  dynamo  may  be  entirely  overcome,  or  even  reversed, 
by  the  addition  of  series  turns. 

When  shunt  dynamos  are  used,  it  is  necessary  to  regulate  the  strength 
of  the  field  magnetism  by  means  of  a  variable  resistance  which  is  con- 
nected into  the  field  circuit  as  is  shown  in  Figure  134.  This  resistance  is 
often  called  a  Field  Rheostat  or  Hand  Regulator. 

211.  Materials  of  Construction.  —  In  order  that  the  number  of  ampere 
turns  required  to  set  up  the  magnetism  in  a  dynamo  shall  not  be  exces- 
sive, it  is  important  to  make  the  magnetic  reluctance  in  the  path  of  the 
lines  of  force  as  small  as  possible.1  On  account  of  this,  the  magnet  frame 
composing  the  magnetic  circuit  of  the  field  is  made  substantially  of  iron. 
In  many  machines  good  wrought  iron  is  used  because  its  permeability  is 
greater  than  that  of  cast  iron,  but  cast  iron  costs  less  per  pound  than 
wrought  iron,  so  that  some  manufacturers  use  cast  iron  in  the  fields  of 


FIG.  135. —  Outline  Drawing  of  Dynamo. 

their  machines.  In  this  case  a  greater  weight  of  cast  iron  is  used  to 
make  up  for  its  lower  permeability,  but  on  account  of  the  smaller  cost  of 
cast  iron  the  heavier  machines  may  not  be  any  more  expensive  than  the 
lighter  ones  in  which  wrought  iron  is  used.  Figure  135  shows  a  very 
common  form  of  machine  in  which  the  fields  are  made  of  wrought  iron, 
with  the  exception  of  the  yoke,  which  is  of  cast  iron. 

In  many  dynamos  and  motors  the  magnet  frames  are  made  of  very 
soft  steel  castings.  This  metal  has  fine  magnetic  qualities,  and,  there- 
fore, is  specially  excellent  for  use  where  light  weight  is  important.  The 

1  Articles  130,  131,  205,  and  206. 


230 


ELECTRICITY   AND    MAGNETISM 


FIG.  136.  —  A  Street  Car  Motor  with  Upper 
Half  of  Frame  raised  to  show  Armature. 


field  of  the  great  2000  horse  power  dynamo  which  was  used  to  furnish 
current  to  the  electric  motors  of  the  Intramural  Railway  at  the  World's 
Fair,  and  which  is  now  furnishing  current  to  electric  street  car  motors, 

is  made  of  steel.  Figure  136 
shows  a  street  railway  motor 
with  a  steel  magnet  frame. 

Armature  cores  are  made  of 
disks  punched  from  thin  sheets 
of  soft  steel  or  wrought  iron, 
and  are  held  to  the  shaft  by 
clamps  and  keys. 

Not  only  does  the  material 
from  which  the  frame  of  a  dy- 
namo is  made  depend  to  some 
extent  upon  the  use  for  which 
the  machine  is  intended,  but  the 

form  of  the  machine  is  also  a  matter  of  choice  which  depends  to  a 
considerable  extent  upon  the  purpose  for  which  it  is  to  be  used.  For  in- 
stance, the  motor  shown  in  Figure  136  is  iron-clad,  that  is,  the  steel  frame 
surrounds  the  field  windings 
and  armature.  This  arrange- 
ment protects  the  windings 
from  danger  of  mechanical 
injury,  and  from  the  danger 
of  being  splashed  by  water 
thrown  by  the  car  wheels 
from  puddles  in  the  street. 
Water  will  quickly  ruin  the 
insulating  qualities  of  the  cot- 
ton thread  and  canvas  which 
are  largely  used  to  insulate 
the  wires  on  dynamos  and  FlG  I37.  _  Four.pole  Dynamo. 

motors. 

The  very  best  copper  wire,  either  round  or  rectangular  in  form,  is 
used  for  winding  armatures  and  fields.  This  wire  is  covered  with  a 
double  or  triple  covering  of  raw  cotton  thread.  When  the  wire  is  in 


DIRECT  CURRENT   DYNAMOS  AND   MOTORS  231 

place,  it  is  varnished  and  baked.  To  keep  the  wires  from  contact  with 
the  iron,  and  one  wire  from  another,  mica,  vulcanized  paper  fibre, 
asbestos,  oiled  paper,  fuller  board,  shellacked  canvas,  and  various  other 
materials  are  used.  For  insulating  the  segments  of  the  commutator  from 
each  other,  mica  is  used.  Great  care  must  be  exercised  in  thus  insulating 
the  parts  of  commercial  dynamos,  as  the  large  amount  of  heat  generated 
and  the  tendency  of  the  wires  to  chafe  against  one  another  is  apt  to  cause 
short  circuits,  with  the  resultant  injury  or  Burning  Out  of  the  machines. 
212.  Multipolar  and  Consequent  Pole  Dynamos.  —  The  dynamo  shown 
in  Figure  137  has  a  field  with  four  poles,  and  that  shown  in  Figure  138 
has  a  field  with  twelve  poles.  These  are  called  Multipolar  to  distinguish 
them  from  two-pole,  or  Bipolar,  machines.  Multipolar  machines  may  have 


FIG.  138.  —  Twelve-pole  Dynamo. 

any  number  of  pairs  of  poles  which  their  dimensions  will  admit.  The 
armatures  for  multipolar  machines  are  wound  upon  the  same  principles 
as  those  used  in  bipolar  machines,  which  have  been  explained. 


232 


ELECTRICITY  AND   MAGNETISM 


Figure  139  gives  a  diagram  of  a  four-pole  machine.    The  arrows  indi- 
cate the  magnetic  circuits,  of  which  there  are  four ;  and  there  are  also 

four  parallel  paths  in  which 
the  current  may  flow  through 
the  armature.  The  number 
of  sets  of  brushes  required 
to  take  the  current  from  the 
commutator  of  a  multipolar 
machine  is  commonly  equal 
to  the  number  of  poles,  but 
sometimes  certain  special 
connections  are  made  in 
the  armature,  which  make 
it  possible  to  use  only  two 
Ujj  I\  sets  of  brushes. 

FIG.  139.  -  Diagram  of  Four-pole  Dynamo.  A  machine  having  the  f°rm 

shown  in  Figure  140  is  often 

spoken  of  as  a  Consequent-pole  machine,  because  the  lines  of  force 
appear  to  enter  the  armature  from  the  centre  of  the  frame. 

Nearly  all  dynamos  and  motors  have  forms  which  are  simply  vari- 
ations of  those  shown  in  this 
article  and  the  preceding 
one.  Dynamo  electromag- 
nets always  have  an  even 
number  of  poles,  since  mag- 
net poles  always  go  in  pairs. 

213.  Dynamo  Connections. 
—  When  a  dynamo  is  started 
for  the  first  time,  it  is  neces- 
sary to  magnetize  its  fields 
from  some  other  machine. 
The  iron  usually  holds  suffi- 
cient residual  magnetism  l  to 

FIG.  140.  — Consequent-pole  Dynamo. 

thereafter  start  the  machine 

into  operation,  and  whenever  started  it  will  quickly  build  up  its  mag- 

1  Article  125. 


DIRECT  CURRENT   DYNAMOS  AND   MOTORS 


233 


netism  to  full  strength.     In  order  that  a  dynamo  may  properly  mag- 
netize itself,  it  is  necessary  that  the  field  windings  be  connected  to 
the  brushes,  so  that  the 
current  generated  by  the 
residual    magnetism   will 
pass  around  the  fields  in 
the  proper  direction.    If 
the  connections  are  made 
properly,  but  the  direc- 
tion of  rotation  of  the  ar- 
mature is  then  reversed, 
the  connections  must  also 
be  reversed.     This  is  il- 
lustrated  in    Figure    141,     FIG.  141.  —  Diagram  showing  the  Connection  of  Brushes 
i_.  t      i  .1        j-rr  an<i  Field  Windings  for  Both  Directions  of  Opera- 

which   shows    the  differ-         tion 

ence  in  the  connections 

of  a  shunt  dynamo  when  the  direction  of  the  armature  rotation   is 

reversed. 

214.  Dynamo  Brushes,  and  their  Proper  Position.  — The  most  impor- 
tant detail  to  look  after,  when  a  direct  current  dynamo  is  in  operation, 
is  the  condition  and  position  of  the  brushes.  Dynamo  and  motor 
brushes  are  sometimes  made  of  copper,  in  which  case  a  bunch  of 

copper  wires,  or  a  number  of  thin  copper 
sheets,  carefully  laid  up  together  and  sol- 
dered at  one  end,  are  commonly  used,  as 
are  also  brushes  woven  of  fine  copper  or 
bronze  wire.  Copper  brushes  usually  touch 
the  commutator  on  a  bevel  (Fig.  142). 
Sometimes  carbon  brushes  are  used.  These 
are  usually  blocks  of  copper-plated  carbon, 

which  touch  the  commutator  either  on  a  bevel  or  radially.     The  brushes 
are  held  against  the  commutator  by  means  of  spring  Brush  Holders. 

When  in  proper  position,  they  are  exactly  opposite  each  other  on  a 
certain  diameter  of  the  commutator  of  a  two-pole  machine.  With  the 
brushes  in  the  proper  position,  a  good  machine  will  usually  deliver  its 
current  with  little  or,  no  Sparking,  while  the  machine  may  spark  badly 


B 


FIG.  142.  —  Position  of  Brushes 
BB  on  Commutator. 


234  ELECTRICITY  AND    MAGNETISM 

if  the  brushes  are  in  any  other  position.  Sparking  is  highly  undesirable, 
because  it  tends  to  destroy  the  commutator.  The  position  of  no  sparking 
may  change  with  the  load  on  the  machine,  in  which  case  the  brushes  on 
a  generator  must  be  moved  forward  as  the  load  increases,  and  the 
brushes  on  a  motor  must  be  moved  backward  under  the  same  condi- 
tions. In  a  generator,  the  proper  position  of  the  brushes  is  slightly  in 
advance  of  a  plane  passed  between  the  pole  tips,  and,  in  a  motor, 
slightly  behind  this  plane. 

215.  Features  required  for  a  Good  Dynamo.  —  The  points  required  in 
a  good  generator  or  motor  for  general  use  are  :  a  powerful  magnetic  field, 
which  requires  a  small  magnetic  reluctance  in  the  magnetic  circuit ;  as 
little  waste  of  power  by  heating  as  possible,  which  requires  that  the 
windings  shall  be  well  designed,  and  that  a  good  quality  of  well  lami- 
nated iron  shall  be  used  in  the  armature  core ;  thorough  insulation 
of  the  windings  to  prevent  contact  with  the  iron  cores,  and  of  the 
various  turns  of  the  windings  from  each  other.  A  neat,  plain  finish  is 
of  advantage  in  an  electrical  machine,  because  it  generally  shows  a 
good  quality  of  workmanship,  which  is  always  necessary  to  produce 
satisfactory  machinery.  A  good  finish  is  also  desirable  because  it 
quickly  shows  dirt  and  bad  treatment,  and  thus  makes  evident  any 
neglect  on  the  part  of  the  dynamo  attendant.  Dirt  and  dampness  are 
two  great  enemies  to  the  insulation  of  dynamos,  and  the  machines  must, 
therefore,  be  kept  perfectly  clean  and  dry,  in  order  that  they  may 
operate  well  and  last  indefinitely  without  unnecessary  repairs. 

QUESTIONS 

43.  On  what  principle  does  a  motor  work  ?    On  what  principle  does  a  dynamo  work  ? 

44.  May  a  dynamo  be  used  as  a  motor  or  a  motor  as  a  dynamo? 

45.  Does  a  motor  generate  a  pressure  in  its  windings  in  the  same  manner  that  a 
dynamo  does? 

46.  What  direction  has  the  counter  electric  pressure  of  a  motor  compared  with 
that  of  the  pressure  used  to  send  current  through  its  windings? 

47.  Why  must  the  power  applied  to  a  dynamo  armature  be  increased  if  the  cur- 
rent generated  is  increased? 

48.  Why  must  the  current  flowing  through  a  motor  armature  increase  if  the  load 
on  the  pulley  is  increased? 

49.  Why  must  the  speed  of  a  motor,  which  has  a  constant  field,  slow  up  a  little 
when  the  load  on  the  pulley  is  increased? 


DIRECT  CURRENT  DYNAMOS   AND  MOTORS  235 

50.  Can  a  motor  be  made  that  does  not  develop  a  counter  electric  pressure  ? 

51.  Tell  how  the  law  of  the  Conservation  of  Energy  applies  to  the  work  put  into 
and  obtained  from  a  motor. 

52.  The  current  that  flows  through  a  motor  armature  is  proportional  to  the  differ- 
ence of  what  pressures? 

53.  What  are  the  causes  of  the  power  losses  in  a  dynamo  or  motor? 

54.  What  is  meant  by  the  efficiency  of  a  dynamo? 

55.  What  is  meant  by  the  efficiency  of  a  motor? 

56.  Of  what  use  is  residual  magnetism,  when  a  dynamo  is  started? 

57.  What  is  series  winding? 

58.  What  is  shunt  winding? 

59.  What  is  compound  winding? 

60.  Why  are  the  windings  of  a  series  field  composed  of  few  turns  of  large  wire? 

61.  \Vhy  are  many  turns  of  small  wire  used  on  a  shunt  field? 

62.  For  what  purposes  are  series-wound  machines  ordinarily  used? 

63.  For  what  purposes  are  shunt  and  compound  wound  machines  ordinarily  used? 

64.  \Vhy  does  the  pressure  of  a  series  dynamo  increase  with  the  current? 

65.  Why  does  the  pressure  of  a  shunt  dynamo  decrease  with  the  current? 

66.  How  can  a  compound  dynamo  be  made  to  give  a  constant  pressure? 

67.  Will  the  pressure  of  a  series  dynamo  increase  with  the  current  after  the  iron 
of  the  magnetic  circuit  has  become  saturated? 

68.  What  is  a  field  rheostat?     What  is  it  used  for? 

69.  What  kind  of  materials  are  dynamo  magnetic  circuits  made  up  of  ? 

70.  Why  are  the  conducting  windings  of  dynamos  made  of  copper? 

71.  What  insulating  materials  are  used  in  dynamos? 

72.  Why  must  dynamos,  or  motors,  be  kept  dry? 

73.  What  is  an  iron-clad  machine? 

74.  What  is  a  multipolar  dynamo?     A  bipolar? 

75.  What  is  a  consequent-pole  machine? 

76.  Why  must  the  field  coil  terminals  be  reversed,  when  the  direction  of  rotation 
of  a  dynamo  is  reversed? 

77.  Would  any  pressure  be  generated  by  a  machine,  if  its  direction  of  rotation 
were  reversed,  but  its  field  coil  terminals  remained  the  same? 

78.  What  is  the  shape  and  material  of  dynamo  brushes? 

79.  What  will  happen  if  the  brushes  are  not  placed  in  the  right  position? 

80.  What  is  the  right  position  for  dynamo  brushes? 

81.  \Vhat  effect  may  change  of  load  have  on  the  sparkless  position  of  dynamo  and 
motor  brushes? 

82.  What  are  the  important   features  in  the  construction  and   operation  of  a 
dynamo  ? 


CHAPTER  XVI 

ALTERNATING  CURRENTS  AND  ALTERNATING  CURRENT 
MACHINERY 

216.  Direct  and  Alternating  Currents  obey  the  Same  Laws.  —  A 

deeply  rooted  belief  seems  to  have  been  cultivated  in  the  minds  of 
many,  that  phenomena  connected  with  the  flow  of  continuous  electric 
currents  and  of  alternating  electric  currents  are  almost  entirely  unrelated. 
This  popular  idea,  however,  is  erroneous  ;  the  principles  which  relate  to 
the  flow  of  electric  currents,  whether  direct  or  alternating,  and  which 
are  applied  to  the  design  and  construction  of  machines  and  circuits,  are 
one  and  the  same.  It  is  desirable,  therefore,  before  taking  up  the 
subject  of  this  chapter,  to  give  a  few  simple  illustrations  for  the  purpose 
of  showing  how  the  fundamental  laws  which  have  been  treated  in 
previous  chapters  apply  equally  to  electric  currents  of  all  characters. 

When  Oersted,  in  1820,  made  known  his  signal  discovery  that  an 
electric  current  exerts  a  magnetic  influence  in  the  space  around  it,  the 
foundation  was  begun  for  our  knowledge  of  the  laws  of  the  flow  of 
alternating  currents.  Within  a  dozen  or  fifteen  years  thereafter  much 
knowledge  of  the  electric  current  had  been  thrashed  out  experimentally 
by  men  like  Ampere,  Arago,  Faraday,  and  Henry.  And  the  last  two 
laid  the  finishing  stone  on  the  foundation  by  searching  out  and  making 
known  the  laws  of  electromagnetic  induction. 

The  apparent  flow  of  electric  current  may  be  likened  to  the  flow  of  a 
fluid,  and  it  may  be  either  Continuous,  Pulsating,  or  Alternating. 

217.  Continuous  Currents  compared  to  Flow  of  Water.  —  The  first 
is  analogous  to  the  flow  of  an  unbranched  river  through  its  channel,  in  a 
season  of  uniform  flow.     The  water  flows  continuously  onward  without 
pause   or  hesitation.     The  velocity  of  the   stream   is   affected  by   the 
character  of  the  banks  and  the  contour  of  the  country  traversed ;  but 

236 


ALTERNATING  CURRENTS  AND  MACHINERY 


237 


the  onward  motion  of  the  volume  of  water  never  ceases,  and  the  quantity 
of  water  flowing  past  any  cross  section  of  the  channel  is  always  the 
same,  in  a  given  time,  though  its  width,  depth, 
and  velocity  may  change  with  the  character  of 
the  channel.  We  may  represent  the  flow  in 
a  graphic  manner  in  this  way :  Suppose  dis- 
tances measured  on  the  vertical  from  a  zero  or 
horizontal  line  represent  the  quantity  of  water 
which  each  minute  flows  past  some  point  along 
the  river.  Then  a  vertical  line  one  inch  long 
(see  Fig.  143),  we  will  say,  means  that  1000 
gallons  of  water  pass  the  point  in  every  minute. 

Now  suppose  a  number  of  measurements  are  made  through  twenty-four 
hours,  and  the  clock  times  when  the  measurements  are  taken  are  recorded. 
Then  we  have  a  number  of  vertical  lines,  each  one  inch  long,  for  every 
1000  gallons  per  minute,  to  represent  the  amount  of  water  flowing  at 


SCALE  FOR  TIME 

FIG.    143.  —  Illustration    of 
Rectangular  Coordinates. 


12          1          2 

FIG.  144.  —  Division  of  Horizontal  Coordinate  into  Parts  representing  Time. 


456789  etc. 
Time 


each  instant  through  the  day  and  night.  We  can  now  extend  our  chart 
and  divide  our  horizontal  line  of  zero  quantity  into  parts  each  represent- 
ing one  hour,  as  in  Figure -144.  And  at  each  of  the  clock  times  set  down, 
we  may  erect  the  vertical  which  has  a  length  corresponding  to  the 


.12  123456  39  ETC.   O'CLOCK 

FIG.  145.  —  Illustration  of  Graphical  Record  of  Measurements. 

quantity  of  water  flowing.     This  river  is  continuous  in  its  flow,  and  all 
the  verticals  are,  therefore,  of  equal  height,  as  in  Figure  145. 

We  may  take  our  observations  of  flow  as  frequently  as  we  choose,  and 
erect  the  corresponding  verticals  at  the  points  corresponding  to  the 


238 


ELECTRICITY   AND   MAGNETISM 


clock  times  of  observations.  Finally,  by  drawing  a  line  through  the  tops 
of  the  verticals  we  have  a  chart  which  shows,  by  the  vertical  height  of 
this  line,  the  rate  of  flow  at  any  time  during  the  twenty-four  hours 
(Fig.  146). 


LINE  SHOWING  QUANTITY  OF  WATER  FLOWING 


A" 

ANY  Tl 

dE  DURI 

G  24  HI 

S. 

9    ETC.   O'C'LOCX 


FlG.  146.  —  Graphical  Record  of  Constant  Current. 

In  the  case  we  are  considering  (that  of  continuous  flow)  the  line 
drawn  through  the  ends  of  the  verticals  is  a  horizontal  line,  that  is,  the 
chart  shows  that  the  flow  of  the  water  is  uniform. 

218.  Pulsating  Currents  compared  to  Liquid  Flow.  —  ^pulsating 
current  may  be  likened  to  the  flow  of  arterial  blood.  With  each  heart- 
beat the  blood  rushes  forward  and  then 
slackens  in  velocity,  and  then  again 
rushes  forward  as  the  heart  beats  again. 
Our  chart,  which  shows  the  quantity  of 
blood  flowing  through  the  artery  at  each 
instant,  is  in  this  case  composed  of  a 
wavy  line  which  never  crosses  the  zero 
line,  as  is  shown  in  Figure  147. 

The  horizontal  scale  now,  instead  of  being  made  in  hours,  may  be 
more  conveniently  made  in  seconds  or  fractions  of  a  second,  since  the 
blood  pulsations  come  many  times  per  minute ;  and  the  vertical  scale 
may  be  made  to  represent  a  flow  in  fractions  of  a  fluid  ounce  per  second 
instead  of  gallons  per  minute,  because  of  the  limited  amount  of  blood 
that  flows  in  an  artery.  The  vertical  height  of  the  wavy  line  in  the 
chart  (above  the  horizontal  scale  line)  still  shows  the  amount  of  blood 
flowing  at  each  instant,  corresponding  to  the  times  read  on  the  horizontal 
scale.  The  Frequency  of  the  heart-beats  is  the  number  of  pulsations 
made  per  minute,  which  is  not  far  from  70  in  the  average  human  adult, 
and  the  duration,  or  Period,  of  the  pulsations  is,  therefore,  not  far  from 


HORIZONTAL  SCALE  SHOWS  TIME  ELAPSED. 


FlG.  147.  —  Graphical   Record  of 
Pulsating  Current. 


ALTERNATING   CURRENTS  AND   MACHINERY  239 

y1^  of  a  minute.  The  period  is  represented  on  the  chart  by  the  time 
that  has  elapsed  between  two  like  points,  as  the  two  points  of  greatest 
flow,  a  and  b. 

219.  Alternating  Currents  compared  to  Flow  in  Tideway,  —  Finally, 
the  alternating  current  may  be  likened  to  the  flow  of  water  in  a  narrow 
tideway.  As  the  tide  rises,  the  water  rushes  up  the  channel  until  near 
high  tide,  when  the  flow  gradually  ceases,  turns,  and  then  with  increasing 
flow  the  water  rushes  down  the  channel  until  near  low  tide,  when  its 
outward  flow  gradually  ceases,  turns,  and  with  increasing  flow  the  water 
begins  to  rush  up  the  channel  again.  The  action  is  repeated  again  and 
again  as  the  days  pass  by.  The  period  of  the  complete  action,  or  Cycle, 
is  little  over  twelve  hours,  and  the  frequency  is,  therefore,  nearly  two 
periods  per  day.  We  can  represent  this 
alternating  flow,  or  current,  also  by  a 
chart,  as  shown  in  Figure  148. 

Assuming    the    period    to    be    exactly 
twelve   hours   (which   is  near  enough   for 

the  analogy),  and  taking  a  day  in  which  „          „      .  (Bj 

low  tide  occurs  at  12  o'clock,  noon,  then,  d 

at  this  time  (12  o'clock,  noon),  no  flow  is   Fl^  ^.-Graphical   Record   of 

One  Cycle  of  Alternating  Current 
occurring  ;  a  little  later  the  flow  is  up  the       Of  Tidal  Flow. 

channel  as  the  tide  rises,  and  the  rate  of 

flow  increases  for  a  time.  This  portion  of  the  tidal  period  is  represented 
by  the  portion  of  the  curve  between  a  and  b  in  Figure  148.  It  is  to  be 
borne  in  mind  that  the  vertical  height  of  the  curve  shows  the  amount 
of  water  flowing  per  minute  at  the  instant  considered  (not  the  height  of 
the  water).  The  flow  continues  up  the  channel  for  a  further  length  of 
time,  but  at  a  decreasing  rate,  until  high  tide  is  reached  at  6  P.M. 
Then,  for  an  instant,  there  is  no  flow  of  the  water.  In  representing  this, 
our  curve  (dropping  down  from  b)  crosses  the  line  of  zero  flow  at  the 
point  marked  c,  which  corresponds  to  the  instant  of  no  flow  at  the  time 
of  high  tide. 

Half  a  tidal  period  has  now  been  completed ;  the  tide  has  reached  its 
flood,  and  begins  to  fall,  and  the  flow,  therefore,  reverses  and  runs  out- 
wards. Since  the  flow  inward  or  up  the  channel  is  shown  on  the  chart 
by  a  vertical  distance  above  the  line  of  zero  flow,  it  is  natural  to  repre- 


240 


ELECTRICITY  AND   MAGNETISM 


FIG.  149.  —  Graphical  Record  of  Two 
Cycles  of  Alternating  Current. 


sent  the  outward  flow  by  a  vertical  distance  below  the  line.     After  the 

turn  of  the  tide,  the  amount  of  flow  increases  for  a  time  up  to  the  maxi- 
mum, and  then  decreases  as  the  low  tide 
is  approached.  This  is  represented  by 
the  curve  from  c  through  d  to  a'.  At  the 
latter  point  low  tide  has  been  reached, 
and  an  entire  cycle  of  the  tide  has  been 
completed,  and  is  represented  on  the 
chart.  The  chart  might  be  continued 

indefinitely,  representing   the   cycles  of  successive  periods  if  desired, 

as  in  Figure  149. 

220.  Phase  of  Flow.  —  It  is  well  known  that  the  character  of  the  tidal 
flow  is  greatly  affected  by  the  character  of  the  channel.     For  instance, 
in  a  narrow,  crooked  channel  the  phase  of  the  flow  is  retarded  as  one 
proceeds  along  its  length,  through  the  buffeting  action  of  the  banks  ;  and 
the  times  of  high  and  low  tides,  when  the  flow  in  the  channel  is  zero, 
may  not  correspond  with  the  times  of  similar  tidal  phases  in  some  other 
channel  or  in  the  open  sea.     In  this  case  we  may  say  that  the  tide  in 
one  channel  Differs  in  Phase  from  that  in  the  other  or  that  in  the  sea ; 
and  a  chart  may  be  drawn  to  represent  the  respective  cycles  of  sea  and 
channel  tides  at  certain  selected  points,  as  in  Figure  150.     In  this  figure 
the  tidal  cycle  in  the  channel  is  shown  to  be 

Retarded,  or  behind  the  tidal  cycle  of  tlie  sea. 
Alternating  currents  of  electricity,  flow- 
ing in  branch  circuits,  may  be  at  different 
phases,  and  they  may  be  represented  on 
a  chart  entirely  similar  to  that  of  Figure 
150.  The  currents  are  said  to  be  Out  of 
Phase,  and  may  be  said  to  be  in  advance 
of  or  behind  each  other,  depending  upon 

which  is  looked  upon  as  the  datum  for  comparison,  —  exactly,  for  in- 
stance, as  we  may  with  equal  propriety  and  the  same  meaning  say 
either  that  the  channel  tide  is  behind  the  tide  of  the  open  sea,  or  that 
the  tide  of  the  open  sea  is  in  advance  of  the  channel  tide. 

221.  Summing  Up.  —  To  recapitulate,  the   electric   current  may  be 
Continuous,  Pulsating,  or  Alternating.     The  first  is  likened  to  a  continu- 


FIG.  150.  —  Graphical  Record  of 
Two  Alternating  Currents  which 
differ  in  Phase. 


ALTERNATING   CURRENTS  AND   MACHINERY 


241 


ugal  Pump,  set- 
ting up  Contin- 
uous Current  of 
Water. 


ous  flow  of  a  river ;  the  second,  to  the  pulsating  flow  of  arterial  blood ; 
and  the  third,  to  the  alternating  flow  of  water  in  a  tideway.  Continuous 
and  pulsating  currents,  that  is,  currents  which  flow  continuously  in  one 
direction,  are  called  Direct  Currents. 

222.  Electric  Current  Flow  compared  to  the  Flow  of  Water  from 
Pumps.  —  We  may  give  another  set  of  analogies  so  as  to  emphasize  the 
relations  still  more  decidedly. 

1.  A  continuous  current  is  like  the  uniform  current 
of  water  set  in  motion  by  means  of  a  centrifugal  pump 
operated  at  a  constant  speed  (Fig.  151). 

2.  A  pulsating  current  is  like  the  current  of  water 
set  in  motion  by  a  piston  pump.    As  the  piston  moves 

forward   in   the  water  cylinder  the  water  therein  is    Fia  I5I  _  Centrif_ 

forced  to  flow  through  the  delivery  pipe.     When  the 

piston  reaches  the  end  of  its  stroke  the  flow  slackens 

or  ceases,  and,  as  the  piston  returns  on  the  stroke, 

the  flow  again  proceeds  as  before  through  the  delivery 

pipe,  and  slackens  as  the  piston  reaches  its  initial  position  (Fig.  152). 

This  is  repeated  as  the  stroke  is  repeated, 
and  the  action  causes  a  succession  of 
impulses  to  the  water,  with  intervening 
pauses  or  slackening  of  the  current. 

3.  An  alternating  current  is  like  the 
current  of  water  which  would  be  set  up 
in  case  the  delivery  and  suction  pipes  of 
the  piston  pump  were  connected  directly 

together,  and  the  valves  removed.     Now,  as  the  piston  moves  back  and 

forth,  the  water  flows  unceasingly  back  and  forth,  alternately  from  one 

end  of  the  cylinder  to 

the   other,    as    long    as 

the    pump   is   operated 

(Fig.   153). 
Figure      153      shows 

clearly  that  a  complete 

cycle  of  the  alternating 

current  is  produced  with 

R 


FIG.  152 . —  Piston  Pump,  setting  up 
Pulsating  Current  of  Water. 


FIG.  153.  —  Piston  Pump  with  By-pass,  setting  up  Alternat- 
ing Current  of  Water. 


242 


ELECTRICITY   AND   MAGNETISM 


each  revolution  of  the  pump-driving  shaft,  that  is,  with  each  360  degrees 
of  angular  motion  of  the  shaft.  We  may,  therefore,  for  the  sake  of  con- 
venience, divide  the 
horizontal  zero  line 
or  axis  in  our  charts 
into  360  parts  for  each 

K'    ^ ^    **>*   1*°°    i*c  «»\MV   2*>"  360°    Bio'/fto-    period  of  the  flow,  and 
X     x  /  J       call  the  parts  degrees 

instead  of  fractions  of 
time.      This  is    illus- 


FIG. 


154.  —  Two  Alternating   Currents  with  30   Degrees 
Difference  of  Phase. 


FIG.  155.  -^-  Actual  Curve  of  Al- 
ternating Electric  Current. 


trated  in  Figure  154, 
which  shows   two  al- 

ternating currents  of  different  phases.     We  may  speak  of  these  as  hav- 

ing 30  degrees  difference  of  phase,  or  they  are  30  degrees  apart,  since 

they  cross  the  horizontal  axis  at  points  which  are  30  divisions  or  degrees 

apart. 

223.    Forms    of    Current    and    Pressure 

Curves.  -  -  The  alternating  curves    which 

are  shown  in  the  preceding  figures  are  all 

smooth  curves,  but  actual  waves  of  alter- 

nating electric  currents  and  pressures  are 

usually  more   or  less   irregular   in   outline, 

and  sometimes  they  are  very  irregular  ;  but 

successive  loops  are  ordinarily  similar.      Two  alternating  current  curves, 

the  outlines  of  which  were  determined  by  experimental  means,  are  shown 

in  Figures  155  and  156. 

It  is  also  a  fact  that  alternating  current 
curves  do  not  have  the  same  shapes, 
whenever  any  iron  is  magnetized  by  the 
currents,  which  is  usually  the  case,  as  the 
waves  of  pressure  which  are  applied  to  the 
circuits  to  produce  the  currents.  The 
curve  in  Figure  155  was  set  up  by  a 
smooth  pressure  wave  with  a  form  quite 

Hke    a    sine    cu  whi]e     the     curve    in 

Figure   156  is  a  curve  of  current  which 


a  ° 

3  to 


FIG.  156.  —  Actual  Curve  of  Alter- 

nating  Current  drawn  by  Student 
in  Electrical  Laboratory  at  Uni- 
versity of  Wisconsin.  ' 


ALTERNATING   CURRENTS   AND   MACHINERY  243 

was  set  up  by  a  quite  flat-topped,  steep-sided  pressure  wave.  Chang- 
ing the  iron  or  other  conditions  of  the  circuit  would  produce  changes  in 
these  current  curves,  though  the  pressure  curves  remained  unchanged. 

224.  Electromagnetic  Inertia  or  Self-induction.  —  If  a  heavy  block  is 
suspended  so  that  it  is  perfectly  free  to  move,  and  then  is  struck  a  sharp 
blow,  for  an  instant  it  offers  a  force  which  opposes  the  force  of  the  blow 
almost  as  though  the  block  were  rigidly  fastened.  This  opposing  force 
is  well  kn9\vn  to  be  caused  by  the  Inertia  of  the  block.  Inertia  sets  up 
a  force  which  tends  to  violently  oppose  any  sudden  change  in  the  motion 
of  a  body,  and  when  the  suspended  block  is  started  swinging  it  may 
strike  a  considerable  blow  upon  its  own  account  when  met  by  an 
obstacle. 

When  an  electromotive  force  is  introduced  into  an  electric  circuit,  the 
circuit,  by  a  kind  of  electromagnetic  inertia,  opposes  the  immediate  flow 
of  the  current  (very  much  as  the  inertia  of  the  suspended  block  opposes 
the  force  of  a  blow  before  the  block  moves),  and  the  rise  of  the  current 
in  the  circuit  is  retarded.  If  the  circuit  is  severed  or  broken  while  cur- 
rent is  flowing,  the  electromagnetic  inertia  makes  an  effort  to  uphold  the 
current  (as  the  swinging  block  is  difficult  to  stop),  and  an  electric  spark 
appears  as  its  evidence  in  the  gap  between  the  severed  ends  of  the  wire. 

Joseph  Henry  originally  discovered  the  cause  of  this  retarding  effect 
about  1832,  and  Faraday  (whose  name  is  almost  a  household  word  on 
account  of  his  discoveries  in  natural  philosophy,  and  especially  in  elec- 
tricity and  magnetism)  was  well  acquainted  with  it  as  early  as  1835,  and 
describes  it  in  his  "  Experimental  Researches."  He  says  :  "  Returning 
to  the  phenomena  in  question,  the  first  thought  that  arises  in  the  mind 
is  that  the  electricity  circulates  with  something  like  momentum  or  inertia 
in  the  wire,  and  that  thus  a  long  wire  produces  effects,  at  the  instant  the 
current  is  stopped,  which  a  short  wire  cannot  produce.  Such  an  ex- 
planation is,  however,  at  once  set  aside  by  the  fact  that  the  same  length 
of  wire  produces  the  effects  in  very  different  degrees,  according  as  it  is 
simply  extended,  or  made  into  a  helix,  or  forms  the  circuit  of  an  electro- 
magnet." He  then  shows  that  the  apparent  inertia  is  due  to  the 
magnetic  effect  of  the  current.  For  instance,  he  says,  "  Further  in- 
vestigation led  me  to  perceive  the  inaccuracy  of  my  first  notions,  and 
ended  in  identifying  these  effects  with  the  phenomena  of  induction 


244  ELECTRICITY   AND    MAGNETISM 

which  I  had  been  fortunate  enough  to  develop  in  the  first  series  of  these 
experimental  researches." 

Faraday  further  speaks  of  this  as  a  retardation  of  the  electric  current 
in  the  circuit,  and  ascribes  the  effect  to  the  "  induction  of  the  current 
itself,"  or  "  self-induction  "  of  the  circuit.1  The  phenomena  of  electro- 
magnetic induction  were  studied  about  1850  (fifteen  years  after  Faraday's 
experiments)  by  Sir  William  Thomson  (now  Lord  Kelvin),  Helmholtz, 
and  other  scientists,  who  brought  to  their  aid  the  powerful  resources  of 
mathematics,  and  their  work  was  canvassed  and  discussed  by  Maxwell 
in  his  book  on  electricity  and  magnetism,  who  showed  that  the  effect  of 
Self-induction  is  truly  the  result  of  Electromagnetic  Momentum,  or  Inertia. 

225.  Lag  of  an  Alternating  Current.  —  The  retardation  of  the  current 
by  electromagnetic  inertia  was  shown  by  Faraday  to  occur  when  the 
current  is  changing  in  value,  and  it,  therefore,  exercises  a  marked  influ- 
ence on  the  ever  changing  alternating  current.     Faraday  showed  that 
the  value  of  the  changing  current  was  retarded,  or  Lagged,  behind  the 
value  which  it  might  be  expected  to  attain,  an  1  which  a  uniform  current 
under  the  same  conditions  would  attain.     We  therefore  know  that  an 
alternating  current  will  lag  behind  the  phase  of  the  alternating  electro- 
motive force  which  causes  it  to  flow,  if  there  is  self-induction  in  the 
circuit.     The  amount  of  the  lag  depends  upon  the  electromagnetic 
character  of  the  circuit.     Thus,  a  straight  wire  causes  less  Retardation, 
or  Lag,  than  the  same  wire  wound  in  a  helix,  because  the  helix  increases 
the  magnetic  effect.     Inserting  an  iron  core  in  the  helix  may  increase 
the  retardation  enormously,  since  the  presence  of  the  iron  again  in- 
creases the  magnetic  effect.     Faraday  said  :  "  If  an  electromagnet  be 
employed,  the  effect  will  be  still  more  highly  exalted,"  as  compared 
with  the  effect  of  the  plain  coil  or  helix  of  wire. 

Faraday's  experiments  were  mostly  carried  on  with  varying  or  pulsat- 
ing currents,  but  later  investigators  took  hold  of  alternating  currents, 
and  much  attention  was  given  to  the  laws  of  flow  of  such  currents  at 
the  time  alternating  current  dynamos  became  known. 

226.  Ohm's  Law  Modified  for  General  Application.  —  We  have  studied 
in  previous  chapters  the  well-known  ratio  generally  called  Ohm's  Law,  in 
which  it  is  asserted  that  a  continuous  current  is  equal  to  the  electrical 

1  Article  142. 


ALTERNATING  CURRENTS  AND   MACHINERY  245 

pressure  upon  a  circuit  divided  by  the  electrical  resistance  of  that  cir- 
cuit. This  so-called  law  is  nothing  more  than  a  special  statement  of  a 
condition  which  may  be  recognized  as  universally  applicable  to  the 
phenomena  of  nature.  The  general  statement  may  be  put  thus  :  The 
result  of  an  effort  is  equal  to  that  effort  divided  by  the  opposing  re- 
sistance. Thus,  for  example,  if  we  stretch  an  elastic  material,  the 
amount  of  stretch  depends  upon  the  ratio  of  the  pull  to  the  elastic 
resistance  of  the  material  ;  if  we  try  to  push  a  heavy  block  along  the 
floor,  the  velocity  of  the  block  depends  upon  the  ratio  of  the  force 
exerted  to  the  frictional  resistance  opposing  the  motion ;  and  so  we 
could  go  on  indefinitely  illustrating  the  general  applicability  in  nature 
of  this  statement  that  any  result  is  dependent  upon  the  ratio  :  effort 
divided  by  resistance. 

We  then  have  for  the  flow  of  continuous  currents  the  rule  that  current 
flowing  (result}  is  equal  to  pressure  {effort)  divided  by  the  opposition 
to  the  current  flow  (resistance)  ;  but  in  the  case  of  continuous  currents 
there  is  no  opposition  to  the  flow  of  the  current  except  electrical  re- 
sistance (that  is,  the  resistance  which  is  determined  by  the  nature,  tem- 
perature, and  dimensions  of  the  conductor),  whence  we  have  Ohm's  Law 
for  the  flow  of  continuous  currents. 

The  fundamental  law  of  the  flow  of  alternating  currents  follows 
directly  from  what  has  gone  before.  The  alternating  current  flowing 
in  a  circuit  is  equal  to  the  pressure  divided  by  the  opposition  to  the 
flow  of  the  current.  In  this  case  the  opposition  is  made  up  of  two 
parts,  one  the  electrical  resistance  spoken  of  above,  and  the  other  the 
opposition  due  to  electromagnetic  inertia. 

We  have  already  learned  how  an  alternating  current  may  be  produced 
in  an  armature  having  a  single  coil  of  wire  which  is  revolved  between 
two  pole  pieces.1  The  ordinary  alternating  current  dynamo  or  Alternator 
is  designed  on  this  principle,  but  is  usually  constructed  with  a  number 
of  coils  on  the  armature  and  with  an  equal  number  of  poles  in  the  field 
magnets.  In  general  construction  an  alternator  is  similar  to  a  con- 
tinuous current  dynamo,  but  before  we  enter  into  a  discussion  of  the 
detailed  arrangements  it  is  well  to  consider  certain  facts  in  regard  to  the 
alternating  current. 

1  Article  196. 


246  ELECTRICITY  AND   MAGNETISM 


QUESTIONS 

1.  Do  the  same  laws  apply  to  the  flow  of  all  forms  of  electric  currents? 

2.  What  great  discovery  did  Oersted  make? 

3.  What  three  classes  may  electric  currents  be  divided  into? 

4.  Compare  the  flow  of  a  continuous  current  to  the  flow  of  a  river. 

5.  Compare  a  pulsating  current  to  the  flow  of  blood  in  an  artery. 

6.  Compare  alternating  current  to  flow  of  water  in  a  tideway. 

7.  What  are  the  "  frequency  "  and  "period  "  of  a  tidal  current? 

8.  How  can  the  "  phase  "  of  the  flow  of  water  in  a  tideway  differ  in  different 
places? 

9.  Describe  pumps  that  will  give  continuous,  pulsating,  and  alternating  currents. 
10.    What  is  meant  by  the  statement  that  two  alternating  currents  differ  in  phase  by 

45  degrees? 

n.    Are  alternating  current  and  pressure  curves  necessarily  similar  or  regular  in 
form? 

12.  Illustrate  the  effect  of  mechanical  inertia. 

13.  Compare  the  inertia  of  a  block  to  a  similar  property  of  electric  circuits. 

14.  Give  a  brief  historical  account  of  the  discoveries  concerning  self-induction. 

15.  Why  does  self-induction  have  an  especially  important  effect  upon  alternating 
currents? 

16.  If  an  alternating  current  is  sent  through  a  self-inductive  circuit,  will  its  value 
at  any  instant  be  the  same  as  though  the  circuit  were  non-inductive? 

17.  What  would  be  the  effect  upon  the  self-induction  of  a  coil  if  an  iron  core 
were  placed  within  it? 

1 8.  Give  a  number  of  illustrations  of  the  ratio  expressed  in  Ohm's  Law. 

19.  What  is  an  alternator? 

227.    Chemical  Effect  of  an  Alternating  Current.  —  If  a  pulsating  cur- 
rent which  varies  in  value  like  that  represented  in  Figure  157  is  passed 
through  a  voltameter,1  the  amount  of  metal,  cop- 

F    /    \     /     \    n      per  for  instance,  which  is  carried  by  the  current 
/          V          \        from  the  anode  to  the  cathode  is  proportional  to 

the  average  value  of  the  current.    In  other  words, 
FIG.  157.  —  Pulsating 

Current.  the  electrolytic  effect  of  a  pulsating  current  is  de- 

pendent upon   the  average  value  of  the  current. 

The  electrolytic  effect  of  the  pulsating  current  represented  by  Figure 
157  is  the  same  as  that  of  a  uniform  current  the  magnitude  of  which 
is  represented  by  the  height  of  the  line  FG  above  the  line  AE. 

1  Article  157. 


ALTERNATING   CURRENTS  AND   MACHINERY  247 

If  the  current  from  a  single  coil  armature  is  not  commutated,  but  is 
led  into  the  external  circuit  by  means  of  collecting  rings,1  as  is  done  in 
telephone  magnetos,  the  second  loop  of  the  curve  representing  the  cur- 
rent falls  below  the  line  AE,  because  the  current  flows  alternately 
in  one  direction  and  then  in  the  other.  This 
is  shown  in  Figure  158,  where  the  perpen- 
dicular distances  from  the  line  OX  to  the 
wavy  line  are  proportional  to  the  strength  of  °" 
the  current  in  the  circuit  at  each  instant. 

During   the   times   represented   by   the   dis- 

FiG.  158.  —  One  Cycle  of  Al- 
tances  A  C,  etc.,  m  which  the  loops  are  above  ternating  Current. 

the  line  OX,  the  current  is  supposed  to  flow 

in  one  direction,  and  during  the  intervening  times,  C£,  etc.,  in  which 
the  loops  are  below  the  line  OX,  the  current  is  supposed  to  flow  in  the 
other  direction. 

Such  an  alternating  current  can  have  no  electrolytic  effect  in  an  elec- 
trolytic cell  like  the  copper  voltameter  described  in  article  757,  since  the 
electric  current  which  flows  in  one  direction  for  one  instant  flows  in  the 
opposite  direction  for  the  next  instant,  and  consequently  the  voltameter 
plates  are  alternately  anode  and  cathode. 

228.  Heating  Effect  of  an  Alternating  Current;  Instantaneous  Squares. 
—  A  different  relation  exists  in  regard  to  the  heating  effects  of  pulsating 
and  alternating  currents.  It  is  to  be  remembered 
that  the  heating  produced  by  a  continuous  current 
when  it  flows  through  a  circuit  is  equal  to  the  cur- 
rent squared  multiplied  by  the  resistance  of  the  cir- 
cuit." The  heating  produced  by  a  pulsating  current 
is  equal  at  every  instant  to  the  value  of  the  current 

at  that  instant  squared  and  multiplied  by  the  resist- 
FlG.  159.  — Curve  rep-  . 

resenting     Squares    ance  of  tlie  Circuit. 

of  the  Ordinates  of       A  curve  maybe  drawn,  as  shown  in  Figure  159, 

sating  Current   l      ^e  height  of  which  at  each  point  is  equal  to  the 

square  of  the  corresponding  height  of  the   curve 

representing  the  current.     The  height  of  this  curve  of  squares  at  each 

point  is  proportional  to  the  power  expended  in  heating  the  circuit  at 

i  Article  196.  2  Article  112. 


248  ELECTRICITY   AND   MAGNETISM 

the  corresponding  instant.     The  same  total  power  would  be  expended 
on  the  circuit  by  a  continuous  current  whose  square  is  equal  to  the  aver- 
age height  of  the  curve  of  squares. 

In  Figure  160,  the  line  APCQE  repre- 
sents the  curve  of  squares  like  that  already 
shown  in  Figure  159  extended  to  two  loops, 
which  correspond  with  the  two  loops  of  pul- 
sating current  ABODE,  and  the  height  of 
•A  E  the  line  FG  above  OX  represents  the  square 

FIG.   160.  —  Curve  of  Squares     of  the  continuous  current  which  causes  the 
for  Two  Successive  Loops  ,         .        .       ,  ,       . 

of  Pulsating  Current.  same  Bating  in  the  circuit  as  the  pulsating 

current.  The  height  of  the  line  FG  is 

greater  than  the  square  of  the  average  value  of  the  pulsating  current, 
and  consequently  the  heating  effect  of  a  pulsating  current  is  greater 
than  that  of  a  continuous  current  equal  to  its  average  value. 

The  reason  for  the  latter  fact  may  be  easily  seen.  The  squares  of 
numbers  increase  in  magnitude  much  more  rapidly  than  do  the  numbers 
themselves.  For  instance,  6  is  twice  3,  but  the  square  of  6,  or  36,  is 
four  times  the  square  of  3,  which  is  9.  On  account  of  this,  the  aver- 
age of  the  squares  of  different  positive  numbers  is  always  greater  than 
the  square  of  the  average  of  the  numbers.  For  instance,  the  average 
of  2,  5,  and  8  is  15  divided  by  3,  or  5,  and  its  square  is  25.  The 
squares  of  these  numbers  are  respectively  4,  25,  and  64,  which  gives 
an  average  of  93  divided  by  3,  or  31.  Now,  if  we  square  the  values 
of  the  pulsating  current  at  each  instant,  we  have  the  squares  of  a  large 
number  of  values  which  range  from  zero  to  a  maximum,  and  the  average 
of  these  squares  is  greater  than  the  square  of  the  average  of  the  original 
values. 

Since  the  heating  effect  of  a  current  is  entirely  independent  of  its  direc- 
tion, an  alternating  current,  such  as  one  of  those  illustrated  in  Figures  ijj, 
156,  or  158,  expends  exactly  the  same  power  in  heating  a  circuit  of  given 
resistance  as  it  would  if  commii  fated  into  a  pulsating  current. 

229.  Product  of  Current  and  Pressure.  —  When  there  is  no  self- 
induction  or  outside  disturbing  factor  in  a  circuit,  the  power  expended 
in  the  circuit  is  always  equal  to  C  X  E  (current  times  electric  pressure). 
Here,  again,  when  the  pressure  and  resulting  current  are  pulsating  or 


ALTERNATING  CURRENTS   AND   MACHINERY  249 

alternating,  we  have  a  series  of  products  of  values,  the  average  of  which 
is  greater  than  the  product  of  the  respective  averages  of  the  current 
and  pressure. 

The  line  ABCDE,  in  Figure  161,  repre- 
sents the  electric  pressure  applied  in  a  cir- 
cuit, and  AbCdE  the  resulting  current.  At 
each  instant  the  power  expended  in  sending 
the  current  through  the  circuit  is  equal  to  the 
product  of  the  corresponding  heights  of  these 
two  curves.  The  height  of  the  curve  APCQE 

at  each  point  is  equal  to  the  product  of  the    FIG.  161.  —  Alternating  Cur- 
, .        .     .    .          r   .  ,  rent,  Pressure,  and  Power 

corresponding  heights  of  the  current  and  press-          Loops 

ure  curves.     Curve  APCQE  may,  therefore, 

be  called  a  power  curve.     Both  its  loops  are  placed  above  the  line  OX 

because  they  both  represent  power  expended  in  the  circuit. 

The  average  power  expended  in  the  circuit  is  represented  by  the 
height  of  the  line  EG,  which  cuts  off  the  tops  of  the  loops  so  that  they 
will  exactly  fill  up  the  intervening  valleys.  The  height  of  the  line  HJ 
represents  the  product  of  the  average  current  by  the  average  pressure, 
which  is  seen  to  be  less  than  the  average  power  represented  by  the 
height  of  the  line  EG. 

230.  Effective  Current  and  Pressure.  —  When  we  measure  the  value 
of  an  alternating  current  we  desire  to  find  the  value  which,  when  squared 
and  multiplied  into  the  resistance  of  a  circuit,  will  give  the  heating  effect 
of  the  current.  This  is  called  the  Effective  value  of  the  current  or  the 
Effective  Current,  and  it  is  greater  than  the  average  value  of  the  current, 
as  we  have  already  seen. 

In  measuring  an  alternating  electric  pressure  or  electromotive  force 
we  likewise  desire  to  find  the  value  which,  when  multiplied  into  the  effec- 
tive current  which  it  causes  to  flow  through  a  circuit  without  self-induc- 
tion, will  give  the  power  expended  in  the  circuit.  This  is  called  the 
Effective  Pressure  or  Effective  Electromotive  Force,  and  is  larger  than  the 
average  pressure. 

From  the  explanation  given  above,  it  is  seen  that  the  effective  value 
of  an  alternating  current  or  an  alternating  electric  pressure  is  equal  to 
the  square  root  of  the  average  of  all  the  squares  of  the  instantaneous 


250 


ELECTRICITY   AND   MAGNETISM 


values  of  the  current  or  pressure  during  the  time  represented  by  one  loop 
in  the  figures.  The  effective  value  is,  therefore,  often  spoken  of  as  the 
"square  root  of  the  mean  (average]  square."  The  power  in  a  circuit  with- 
out self-induction  is  equal  to  the  product  of  the  effective  current  and 
pressure. 

231.    Alternating  Current  Measuring  Instruments.  —  Since  the  indi- 
cations of  an  electrodynamometer  or  of  a  hot  wire  electrical  measuring 


THERMOMETER  SCALEr60-t20 


FIG.  162.  —  Perspective  View  of  Interior  of  Weston  Alternating  Current  Voltmeter  from 

below. 

instrument  are  proportional  to  the  square  of  the  current  flowing  through 
the  instrument,1  such  instruments  are  excellently  adapted  to  measuring 
alternating  currents.  The  number  of  alternations  made  in  each  minute 
by  the  alternating  currents  which  are  ordinarily  used  is  so  great  that  the 
movable  coil  of  an  electrodynamometer  acts  exactly  as  though  it  were 
pulled  around  by  a  continuous  force  proportional  to  the  average  of  the 
squares  of  the  instantaneous  values  of  the  current.  The  square  root  of  the 

1  Articles  176  and  178. 


ALTERNATING  CURRENTS  AND  MACHINERY 


251 


indication  of  the  instrument  is,  therefore,  proportional  to  the  effective 
value  of  the  alternating  current  flowing  through  its  coils.  One  form  of 
electrodynamometer  which  is  commonly  used  for  measuring  alternating 
currents  is  shown  in  Figure  99.  The  mechanism  of  an  alternating  cur- 
rent voltmeter  made  upon  the  same  principle  is  shown  in  Figure  162.  It 


AAAAAAAMA/VW 

FIG.  i62a. —  Diagram  of  Weston  Alternating  Current  Voltmeter. 


is  evident,  as  these  instruments  measure  continuous  currents  or  pressures, 
or  effective  alternating  currents  or  pressures,  on  the  same  scale,  that  we 
may  consider  the  alternating  values  as  equivalent  to  continuous  currents 
or  pressures  that  have  the  same  heating  effect  in  a  circuit. 

The  construction  of  the  Weston  alternating  current  voltmeter  may  be  understood 
by  following  the  figures,  which  are  lettered  alike.  The  instrument  is  constructed 
on  the  principle  of  the  electrodynamometer.  B  is  the  movable  coil,  and  AA  is  the 
stationary  or  fixed  coil.  M,  N,  O  are  binding  posts.  J,  K  are  extra  resistances.  L 
is  a  special  variable  resistance  used  to  correct  the  readings  for  variations  of  tempera- 
ture. D  is  a  push  button  switch,  and  C,  C'  are  springs  through  which  the  current 
enters  and  leaves  the  movable  coil.  P  is  the  needle  or  pointer,  H  is  the  scale  which 


252  ELECTRICITY   AND   MAGNETISM 

is  engraved  with  two  sets  of  figures,  and  G  is  a  thermometer  with  its  bulb  near  the 
coils  of  the  instrument  and  its  stem  in  view  near  the  scale  of  the  instrument.  The 
instrument  is  usually  boxed  up  so  that  only  the  scale  //,  //  over  which  the  pointer 
moves,  the  dial  of  Z,  and  the  stem  of  the  thermometer  are  visible. 

When  the  instrument  is  in  service,  the  voltmeter  is  connected  to  the  circuit  by 
means  of  the  binding  post  O,  and  either  the  binding  post  M  or  the  binding  post  N. 
When  the  button  D  is  depressed,  current  flows  through  the  fixed  and  movable  coils 
and  through  the  resistance  J  -f-  K  or  J  alone  (depending  upon  which  binding  post  — 
M  or  N —  is  used),  and  the  pointer  is  caused  to  move  over  the  scale  by  the  move- 
ment of  the  movable  coil  which  is  caused  by  the  electromagnetic  attractions  between 
the  current  in  its  windings  and  the  current  in  the  windings  of  the  fixed  coil. 

A  pointer  on  the  dial  L  is  set  at  a  mark  which  corresponds  to  the  temperature 
indicated  by  the  thermometer,  and  more  or  less  of  the  resistance  coils  connected  to 
the  dial  are  thus  included  in  the  voltmeter  circuit.  In  this  way  the  resistance  of  the 
voltmeter,  measured  from  binding  post  to  binding  post,  may  be  kept  uniform,  regard- 
less of  the  temperature  of  the  instrument,  and  the  readings  are  thus  corrected  for 
variations  of  temperature.  The  resistance  of  the  windings  AA  and  B,  added  to  the 
resistance  y,  is  just  equal  to  one-half  of  the  resistance  of  the  same  windings  plus  the 
resistance  of  J  -f  K.  Consequently  only  half  as  much  electrical  pressure  between 
the  instrument  terminals  is  required  to  cause  a  given  movement  of  the  needle  when 
the  binding  posts  O  and  N  are  used,  as  when  the  binding  posts  0  and  M  are  used. 
The  scale  which  reads  up  to  7.5  volts,  in  the  instrument  illustrated,  is  therefore  used 
in  connection  with  binding  posts  0  and  N,  and  the  scale  which  reads  up  to  15  volts 
with  the  binding  posts  O  and  M.  The  movement  of  the  coil  and  pointer  is  opposed  by 
the  springs  CO  1  and  the  scale  is  engraved  so  that  the  instrument  is  direct  reading.2 

Since  hot  wire  instruments  also  average  up  the  squares  of  the  instan- 
taneous values  of  the  current,  they  are  now  used  to  a  considerable  extent 
for  measuring  alternating  currents.  They  are  made  into  voltmeters  and 
shunted  amperemeters3  upon  the  same  principle  as  the  Cardevv  volt- 
meter4 which  has  long  been  used  as  an  alternating  current  instrument. 
Electrostatic  voltmeters 5  also  give  indications,  the  square  roots  of  which 
are  proportional  to  effective  alternating  pressures,  when  the  needle  is 
electrically  connected  to  one  pair  of  quadrants,  as  is  usually  done.  The 
scales  of  all  these  instruments  may  be  so  graduated  as  to  be  direct  read- 
ing. Magnetic  vane  instruments  which  are  described  in  Article  180  also 
are  used  satisfactorily  in  alternating  current  measurements. 

Alternating  current  wattmeters  are  made  upon  the  plan  of  an  electro- 
dynamometer  with  a  coil  of  low  resistance  for  connecting  in  series  with 

1  Articles  174, 175.      2  Articles  179, 182.      3  Article  181.      4  Article  182.      s  Article  183. 


ALTERNATING  CURRENTS  AND   MACHINERY  253 

the  circuit,  and  a  coil  of  high  resistance  for  connecting  across  the  ter- 
minals. These  instruments  are  in  every  respect  the  same  as  wattmeters 
intended  for  use  in  direct  current  circuits  described  in  Article  187. 

232.  Frequency  and  Period  of  an  Alternating  Current.  —  An  alternat- 
ing current  is  said  to  make  as  many  Alternations  per  Minute  as  it  makes 
changes  in  direction  in  each  minute.     Instead  of  speaking  of  the  number 
of  alternations  per  minute  of  an  alternating  current  it  is  quite  common 
and  more  scientific  to  speak  of  its  Frequency,1  that  is,  the  number  of 
double  alternations  made  per  second. 

The  early  alternating  current  dynamos  which  were  generally  used  in 
this  country  furnished  currents  making  from  15,000  to  16,500  alterna- 
tions per  minute,  or  frequencies  of  from  125  to  137.5  periods  per  sec- 
ond ;  but  frequencies  only  half  as  great,  and  even  less,  have  come  into 
use  during  later  years,  and  the  commonest  frequency  used  in  this  country 
is  now  60  periods  per  second  or  7200  alternations  per  minute.  The 
great  power  plant  at  Niagara  Falls  uses  a  frequency  of  25  periods  per 
second. 

The  number  of  alternations  per  minute  is  equal  to  2  x  60,  or  1 20  times 
the  frequency,  since  60  is  the  number  of  seconds  in  a  minute.  The 
fraction  of  a  second  during  which  an  alternating  current  makes  two  loops 
is  called  its  Period.2 

Example  A.  If  an  alternating  current  has  a  frequency  of  100  periods  per  second, 
how  many  alternations  does  it  make  per  minute  ?  Ans.  12,000. 

Example  B.  What  is  the  frequency  of  an  alternating  current  making  6000  alter- 
nations per  minute  ?  Ans.  50. 

Example  C.  What  are  the  periods  in  examples  A  and  B  ?  Ans.  r^  and  -^  of  a 
second. 

233.  Effect  of  Self-induction  on  the  Flow  of  Alternating  Currents.  — 

The  very  important  effects  of  electromagnetic  inertia  on  the  flow  of 
alternating  currents 3  makes  them  appear  to  be  more  complex  than  con- 
tinuous currents.  The  point  is  so  important  that  it  must  be  given  very 
careful  attention. 

When  a  continuous  current  is  passed  through  an  incandescent  lamp, 
the  amount  of  power  expended  by  the  passage  of  the  electric  current 
through  the  lamp  filament,  which  is  converted  into  heat  and  light,  is 

1  Article  219.  2  Article  219.  8  Articles  224,  225,  226. 


254  ELECTRICITY  AND   MAGNETISM 

equal  to  C  x  E.  In  the  same  way,  when  an  alternating  current  is  passed 
through  an  incandescent  lamp,  the  amount  of  power  which  is  expended 
in  the  lamp  filament,  and  converted  into  light  and  heat,  is  also  equal  to 
C  x  £,  where  C  and  E  are  the  effective  values  of  the  current  and  press- 
ure measured  by  the  proper  alternating  current  instruments  which  were 
explained  in  Article  231.  We  therefore  see  that  an  incandescent  lamp 
which  is  intended  to  give  sixteen  candle-power  at  a  pressure  of,  say  no 
volts,  will  be  equally  efficient  when  it  is  connected  to  a  constant  pressure 
circuit  which  furnishes  it  continuous  current  at  a  uniform  pressure  of  no 
volts,  or  when  it  is  connected  to  a  circuit  which  furnishes  it  alternating 
current  at  an  effective  pressure  of  no  volts.  If  the  current  flowing 
through  the  lamp  when  it  is  connected  to  the  continuous  current  circuit 
is  measured  by  an  accurate  amperemeter  of  any  kind,  and  a  measure- 
ment also  is  made  when  the  lamp  is  connected  to  the  alternating  current 
circuit  by  an  accurate  electrodynamometer,  exactly  the  same  amount  of 
current  will  be  found  to  flow  through  the  lamp  in  the  two  cases. 

Now,  suppose  we  take  200  feet  of  No.  7  B.  &  S.  gauge  insulated  copper 
wire.  Its  resistance  is  almost  exactly  one-tenth  of  an  ohm  at  ordinary 
temperatures,  and  it,  therefore,  requires  only  one-tenth  of  a  volt  to  send 
one  ampere  of  continuous  current  through  it.  This  is  true  whether  the 
wire  is  stretched  out  straight,  wound  in  a  simple  coil,  or  wound  around 
an  iron  core,  since  the  resistance  of  the  wire  at  a  given  temperature 
depends  only  upon  its  length,  cross  section,  and  material,1  and  none  of 
these  are  altered  by  coiling  or  winding  up  the  wire. 

To  send  one  ampere  of  alternating  current  of,  say,  a  frequency  of  125 
periods  per  second  (15,000  alternations  per  minute)  through  this  wire 
when  it  is  stretched  straight  out  requires  a  tenth  of  a  volt  effective  press- 
ure, or  the  same  as  in  the  case  of  a  continuous  current.  The  straight 
wire,  therefore,  acts  in  practically  the  same  way  toward  continuous  and 
alternating  currents,  exactly  as  does  the  incandescent  lamp  filament, 
which,  indeed,  is  nothing  more  than  a  bent  wire  made  of  carbon. 

Now,  if  the  wire  is  coiled  up,  a  greater  pressure  than  one- tenth  of  a 
volt  will  be  required  to  send  one  ampere  through  the  wire,  while  if  it  is 
wound  on  a  big  laminated  iron  core  there  may  be  as  much  as  IOO  volts, 
or  even  more,  required  to  send  an  ampere  through  the  wire. 

1  Article  95. 


ALTERNATING  CURRENTS  AND  MACHINERY       .255 

We  know  that  the  resistance  of  the  wire  is  not  changed  by  coiling  it  up 
or  by  winding  it  around  an  iron  core,  so  that  the  actual  resistance  is 
one-tenth  of  an  ohm  all  the  time.  This  is  proved  by  the  fact  that  coil- 
ing the  wire  and  winding  it  around  an  iron  core  does  not  change  the 
amount  of  pressure  required  to  send  one  ampere  of  continuous  current 
through  it.  It  also  may  readily  be  proved  by  measuring  the  resistance 
of  the  wire  by  a  Wheatstone  bridge  when  the  wire  is  stretched  straight 
out  and  when  it  is  wound  on  an  iron  core. 

The  action  of  the  alternating  current  as  thus  seen  might  lead  us  to 
suppose  that  the  flow  of  alternating  currents  does  not  follow  Ohm's  Law.1 
The  flow  of  alternating  currents  does  follow  a  law  like  Ohm's,  however, 
and  the  peculiar  action  described  is  explained  in  the  following  articles. 

234.  Effect  of  Self-induction  is  caused  by  the  Magnetic  Field  created 
by  the  Current.  —  In  Articles  142  and  224  it  is  described  how  either  an 
increase  or  a  decrease  of  current  in  a  coil  is  retarded  by  the  magnetic 
effect  of  the  different  turns  of  the  coil  tending  to  stop  any  change  in  the 
current.  This  effect  is  magnified  to  a  large  degree  when  the  coil  is 
wound  on  an  iron  core,  since  the  iron  largely  increases  the  magnetic 
effect  of  the  turns  and  so  increases  the  self-induction  of  the  coil; 
while  a  wire  stretched  out  straight  or  bent  in  a  hairpin,  like  an  incan- 
descent lamp  filament,  has  very  little  self-induction. 

When  a  battery  is  connected  so  as  to  send  a  current  through  a  straight 
wire  the  current  rises  to  its  full  value,  according  to  Ohm's  Law,  almost 
instantly.  When  the  same  wire  is  coiled  up  and  connected  to  the  bat- 
tery, the  current  does  not  rise  to  its  full  value  instantly  on  account  of 
the  retarding  effect  of  self-induction,  but  the  delay  is  only  a  very  small 
fraction  of  a  second.  Now,  when  the  wire  is  wound  on  an  iron  core 
and  then  connected  to  the  battery,  the  effect  of  self-induction  is  so  great 
that  it  takes  quite  an  appreciable  portion  of  a  second  for  the  current  to 
rise  to  its  full  steady  value.  The  final  steady  value  reached  by  the  cur- 
rent is  not  changed  by  the  self -induction,  but  is  just  the  same  in  each 
case  if  the  pressure  is  uniform,  because  the  self-induction  can  have  an 
effect  only  while  the  current  is  changing  in  value. 

As  explained  in  Article  142,  when  a  current  is  rising  in  a  coil  it  sets 
up  lines  of  force,  which  in  turn  set  up  a  counter-electromotive  force 

1  Article  92. 


256  ELECTRICITY   AND   MAGNETISM 

which  dams  back  the  current ;  but  when  the  current  is  falling,  the  dis- 
appearing lines  of  force  set  up  a  pressure  in  accord  with  the  direction 
of  the  current  which  tends  to  keep  the  current  flowing. 

235.  Current  Lag.1 — An  alternating  current  changes  all  the  time,  so 
that  it  never  has  a  steady  value,  and  the  effect  of  self-induction  is  there- 
fore felt  by  it  all  the  time.     While  the  current  is  rising,  self-induction 
tends  to  hold  it  back  or  keep  it  from  rising,  and  when  the  current  is 
falling,  self-induction  still  tends  to  keep  it  from  changing.     The  result 
is  that  in  a  circuit  having  self-induction  an  alternating  current  is  always 
retarded  a  certain  amount  behind  the  alternating  pressure  which  sets  it 
up.     The  current  is  said  to  have  a  Lag.     This  same  retardation  causes 
the  maximum  value  of  the  current  to  be  smaller  than  it  would  be  were 
there  no  effect  of  self-induction. 

236.  Impedance.  —  We,  therefore,  see  that,  where  an  alternating  cur- 
rent flows  through  a  circuit  which  has  such  a  form  that  its  self-induc- 
tion is  appreciable,  the  alternations  made  by  the  current  come  a  small 
fraction  of  time  later  than  those  made  by  the  electric  pressure,  and  the 
value  of  the  current  is  smaller  than  if  no  self-induction  were  present. 
The  effect  is  exactly  as  though  the  current  loops  of  Figure  161  were  not 
placed  directly  under  the  pressure  loops,  but  were  pushed  a  certain 

small  amount  back  of  the  position  of  the 
pressure  loops.  The  curves  are  so  drawn  in 
Figure  163,  which  is  similar  to  Figures  150 

and  I54' 

The  effect  of  self-induction  in  decreasing 
the  amount  of  alternating  current  which  flows 
FIG.  163.  —  Lagging  Current,     in  a  circuit  depends  upon  the  magnetic  effect 
which  the  different  parts  of  the  circuit  have 

on  each  other,  and  also  upon  the  frequency  of  the  current.  The  same 
result  is  brought  about  as  would  be  given  by  increasing  the  resistance 
of  the  circuit  a  certain  amount.  It  is  therefore  usual  to  speak  of  the 
Apparent  Resistance,  or  Impedance,  of  a  circuit  through  which  an  alter- 
nating current  flows. 

The  effective  current  in  an  alternating  circuit  is  equal  to  the  effective 
electrical  pressure  applied  to  the  circuit  divided  by  the  Impedance  of  the 

1  Article  225. 


ALTERNATING  CURRENTS  AND   MACHINERY  257 

circuit.  This  may  be  called  the  Ohm's  Law  of  the  alternating  current 
circuit. 

The  Impedance  is  a  combination  of  the  tnie  resistance  of  the  wire 
composing  the  circuit  with  the  effect  due  to  self-induction.  The  true 
resistance  of  the  wire  depends  only  upon  its  length,  cross  section,  and 
material,  while  the  effect  of  self-induction  depends  itpon  the  magnetic 
effect  of  the  different  parts  of  the  circuit,  and  upon  the  frequency  of  the 
current. 

When  a  continuous  current  flows  through  a  circuit,  the  true  resist- 
ance of  the  circuit,  as  measured  by  a  Wheatstone  bridge,  only  need  be 
considered,  but  when  an  alternating  current  flows  through  the  same  cir- 
cuit, the  impedance  comes  into  account. 

The  remarkable  results  which  are  brought  about  in  alternating  current 
circuits,  on  account  of  the  current  hanging  back  or  Lagging  behind  the 
electrical  pressure  will  now  be  considered.  Before  entering  upon  this 
subject  the  student  should  study  the  preceding  descriptions  until  he 
gets  a  true  idea  of  the  lagging  of  the  loops  of  an  alternating  current 
behind  the  pressure  which  sets  up  the  current,  and  the  cause  of  this 
lagging. 

QUESTIONS 

20.  Why  is  the  electrolytic  effect  of  a  pulsating  current  dependent  on  its  mean 
value  ? 

21.  Why  does  an  alternating  current  ordinarily  produce  no  electrolytic  effect  in  a 
water  voltameter  ? 

22.  Why  is  the  heating  effect  of  a  pulsating  or  of  an  alternating  current  greater 
than  that  of  a  continuous  current  of  the  same  average  value  ? 

23.  Is  the  heat  produced  by  an  electric  current  when  flowing  through  a  circuit 
affected  by  the  direction  of  its  flow  ? 

24.  Why  is  the  actual  power  in  a  non-inductive  alternating  circuit  greater  than 
would  be  indicated  by  multiplying  together  the  average  values  of  current  and  pressure? 

25.  What  is  meant  by  the  phrase  "  effective  current "  ? 

26.  What  is  meant  by  the  phrase  "  effective  pressure  "  ? 

27.  WThy  are  effective  values  of  currents  and  pressures  used  instead  of  average 
values  ? 

28.  What  is  the  power  in  a  non-inductive  circuit  equal  to,  in  terms  of  effective 
current  and  pressure  ? 

29.  Why  arc  instruments  based  on  the  principle  of  the  electrodynamometer,  or 
on  the  hot-wire  principle,  usually  used  in  measuring  alternating  currents  ? 

S 


258  ELECTRICITY   AND    MAGNETISM 

30.  Why  are  instruments  based  on  the  principles  named  in  Question  29,  or  on  the 
electrostatic  principle,  usually  used  in  measuring  alternating  voltage  ? 

31.  What  would  be  the  effect  of  putting  an  amperemeter  having  a  permanent  mag- 
net in  series  with  an  alternating  current  circuit  ?     Why  would  it  not  give  a  reading  ? 

32.  Can  soft  iron  core  instruments  be  made  so  that  they  can  be  used  on  alter- 
nating current  circuits  ?     Explain  the  action. 

33.  Describe  a  wattmeter  that  may  be  used  on  alternating  current  circuits. 

34.  What  is  meant  by  the  frequency  of  an  alternating  current  ? 

35.  What  is  meant  by  the  period  of  an  alternating  current  ? 

36.  What  frequencies  are  commonly  used  ? 

37.  Will  essentially  the  same  current  flow  through  a  straight  wire  under  a  given 
pressure  whether  it  be  continuous  or  alternating  ? 

38.  Suppose  the  wire  of  37  is  coiled  up  ? 

39.  Suppose  an  iron  core  is  inserted  in  the  coil  of  38? 

40.  What  causes  the  effects  of  self-induction  ? 

41.  Why  is  a  self-inductive  pressure  set  up  in  a  circuit  only  when  the  current  is 
changing  ? 

42.  Why  is  it  that  when  a  steady  pressure  is  impressed  upon  a  self-inductive  cir- 
cuit the  self-induced  pressure  acts  against  the  impressed  pressure  while  the  current  is 
rising? 

43. .  Why  is  it  that  the  induced  pressure  is  in  the  same  direction  as  the  impressed 
pressure  when  the  current  is  falling? 

44.  Explain  why  an  alternating  current  lags  in  a  self-inductive  circuit. 

45.  Is  the  apparent  resistance  which  an  inductive  circuit  offers  to  the  flow  of  an 
alternating  current  greater  than  the  resistance  due  to  the  form  and  material  of  the  wire? 

46.  What  is  impedance  ? 

47.  Give  the  generalized  form  of  Ohm's  Law  as  it  applies  to  the  flow  of  alternating 
currents. 


237.  Power  in  a  Self-inductive  Alternating  Circuit.  —  In  Article  229 
it  was  explained  that  the  power  in  an  alternating  circuit  is  equal  at  any 
instant  to  the  product  of  the  instantaneous  pressure  and  current,  and 
that  its  average  value  is  the  average  of  these  products.  Where  the 
curves  of  pressure  and  current  are  in  the  same  phase,  the  power  loops 
are  as  shown  in  Figure  161,  but  if  the  current  lags  on  account  of  self- 
induction,  as  in  Figure  163,  the  power  loops  are  altered  in  position  and 
become  as  illustrated  in  Figure  164. 

It  is  seen  in  this  figure  that  there  are  large  positive  loops  and  small 
negative  loops.  The  negative  loops  are  located  between  the  points  ab 
and  cdt  where  the  current  and  pressure  curves  are  on  opposite  sides  of 


ALTERNATING   CURRENTS  AND   MACHINERY  259 

the  horizontal  line.  This  means  that  during  part  of  each  half  period 
work  is  being  absorbed  by  the  circuit  as  represented  by  the  positive 
loop,  and  during  another  part  of  the  time 
work  is  being  returned  by  the  circuit  to 
the  source  of  electrical  power.  The  aver- 
age work  absorbed  by  the  circuit  is  now 
the  difference  between  the  average  values 
of  the  positive  and  negative  loops  and  is 
less  than  that  which  would  be  produced 

by  an  equal  current  which  flowed  in  phase 

vi     ,1  FIG.  164.  —  Power  Loops  when 

With   the  pressure.  4  Current  lags. 

If  the  angle  of  lag  happens  to   be  90° 

(in  which  case  the  current  is  halfway  behind  the  pressure  curve),  as  in 
Figure  165,  the  negative  loops  are  then  equal  to  the  positive  loops,  and 

all  the  power  absorbed  by  the  circuit  dur- 
ing one  quarter  period  is  returned  during 
the  next. 

It  is  impossible  to  have  quite  90°  lag  in 
a  practical  circuit  because  some  power  is 

always  required  to  send  a  current  through 
FIG.  165.  —  Power  Loops  when 

Current  lags  Ninety  Degrees.  a  circuit  no  matter  how  small  the  resist- 
ance may  be.  It  must  always  be  remem- 
bered that  the  amount  of  current  flowing  through  a  circuit  at  any  instant 
is,  by  Ohm's  Law,  equal  to  the  pressure  divided  by  the  resistance.  The 
reason  that  in  an  inductive  circuit  the  current  does  not  apparently  follow 
this  law  is  because  the  pressure  acting  to  produce  current  at  each  instant 
is  the  difference  between  the  pressure  impressed  (which  is  shown  in  the 
curves)  and  the  counter-pressure  of  self-induction,  just  as  the  current 
flowing  through  the  armature  of  a  direct  current  motor  is  equal  to  the 
difference  between  the  impressed  and  the  counter-pressures  divided  by 
the  resistance  of  the  armature. 

238.  Measurement  of  Power  in  an  Alternating  Circuit. — When  an 
alternating  current  flows  through  a  circuit  which  does  not  have  any  self- 
induction,  the  current  loops  and  pressure  loops  are  in  unison  as  is 
illustrated  by  the  curves,  AbCdE  and  ABCDE,  in  figure  161.  In  this 
case  we  can  measure  the  power  which  is  used  in  the  circuit  by  an  alter- 


260 


ELECTRICITY   AND   MAGNETISM 


100  VOLT 
CURRENT  SUPPL1 


nating  current  voltmeter  and  an  electrodynamometer,  because  these 
instruments  measure  the  effective  pressure  and  the  effective  current,  and 
the  two  readings  multiplied  together  give  the  power  used  in  the  circuit.1 
We  can,  therefore,  measure  the  power  used  in  an  incandescent  lamp, 
which  is  operated  on  an  alternating  current  circuit  by  means  of  an 
alternating  current  amperemeter  and  an  alternating  current  voltmeter, 
exactly  in  the  same  way  that  we  would  measure  the  power  used  by  it 
when  operated  on  a  continuous  current  circuit. 

If  a  coil  of  wire,  having  an  iron  core,  is  substituted  for  the  incandes- 
cent lamp,  the  current  loops  are  caused,  by  the  effect  of  self-induction, 
to  lag  behind  the  pressure  loops,  and  we  are  not  able  to  measiire  the 
poiver  used  in  the  coil  by  an  amperemeter  and  a  voltmeter,  as  we  did  in 
the  case  of  an  incandescent  lamp,  because,  in  this  case,  the  product  of 
the  effective  current  and  the  effective  pressure  is  not  equal  to  the  power. 
The  actual  power  used  in  the  circuit  is  less  than  the  value  given  by  the 
product  of  the  effective  current  and  pressure.2  At 
EACH  INSTANT  the  power  consumed  in  the  circuit  is 
equal  to  the  product  of  the  current  and  the  pressure 
at  that  instant,  exactly  as  is  the  case  when  the  cur- 

Jf] rent  and  pressure  loops  are  in  unison;  but  when  the 

current  lags  behind  tJie  pressure,  the  TOTAL  POWER  con- 
sumed is  less  than  would  have  been  used  in  sending 
the  same  current  under  the  same  pressure  through  a 
circuit  without  self -induction? 

The  moral  of  this  is :  do  not  try  to  measure  the 
power  used  in  any  alternating  current  circuit,  which 
has  appreciable  self-induction,  by  an  amperemeter 
and  a  voltmeter.  For  instance,  if  the  alternating 

current  flowing  in  the  primary  coil  of  a  transformer 
FIG.  166.  —  Connec- 
tions of  Wattmeter     1S  measured  and  its  value  is  multiplied  by  the  alter- 

for  measuring  Pow-     nating  pressure  which   causes   the   current  to  flow, 
of  Transformer  °       tne  product  does  not  represent  the  power  used  by 

the  transformer. 

The  power  used  when  an  alternating  current  is  caused  to  flow  through 
a  circuit  which  has  self-induction  may  be  measured  by  a  proper  watt- 

3  Article  237. 


TRANSFORMER 


"UIOO  VOLTS' 


1  Article  229. 


'2  Article  237. 


ALTERNATING  CURRENTS  AND  MACHINERY 


26l 


meter ;  such,  for  instance,  as  that  made  out  of  an  electrodynamometer, 
explained  in  Article  187.  The  indications  of  such  a  wattmeter,  when 
connected  to  the  circuit  as  directed  in  Article  187,  are  directly  pro- 
portional to  the  power  used  in  the  circuit,  because  they  are  the  AVERAGE 
of  the  values  of  the  power  given  to  the  circuit  at  every  instant. 

If  it  is  desired  to  find  out  how  much  power  is  wasted  in  the  iron  core 
of  an  alternating  current  transformer,  for  instance,  it  can  be  quickly 
done  by  connecting  up  a  wattmeter  as  shown  in  Figure  166;  for  then,  if 
the  wattmeter  has  been  calibrated,  its  readings  will  at  once  give  the  power. 

239.  Transformers.  —  Alternating  currents  are  widely  used  for  the 
distribution  of  electric  currents  for  the  purpose  of  electric  lighting, 
because  it  is  possible  to  use  a  high  pressure  on  the  distributing  lines  and 
thus  make  a  saving  in  the  expense  of  wires,  and  the  high  pressure  may 


TERMINAL  OF 
PRIMARY 
WINDING 


TERMINALS  OF 
SECONDARY 
•WINDINGS 


TERMINAL  OF 
PRIMARY 
WINDING 


TERMINALS  OF 
SECONDARY 
WINDINGS 


FIG.  167.  —  Skeleton  View  of  Transformer. 

be  reduced  with  little  loss  of  power  by  means  of  induction  coils  or 
transformers  to  a  pressure  which  it  is  safe  to  use  in  houses. 

These  transformers  consist  of  two  coils,  the  primary  and  secondary 
coils,  which  have  well-laminated  iron  cores  made  of  strips  or  "  stamp- 


262 


ELECTRICITY   AND   MAGNETISM 


ings  "  of  thin  wrought  iron  laid  together  in  such  a  manner  that  they 
make  for  the  coils  a  core  with  closed  ends,  thus  affording  a  complete 
magnetic  circuit  for  the  magnetism  set  up  by  a  current  in  the  coils. 
The  primary  coil  usually  consists  of  many  turns  of  small  wire,  while  the 
secondary  coil  consists  of  fewer  turns  of  larger  wire.  The  coils  are  care- 
fully insulated  with  mica,  rubber  insulating  tape,  or  other  insulating 
materials,  and  the  core  is  then  built  up  by  slipping  the  "  stampings  " 
into  position. 

Figure  167  gives  a  skeleton  view  of  a  transformer  in  the  iron  protect- 
ing case,  which  shows  the  positions  of  the  core  and  coils  in  a  clear  man- 
ner. Figure  168  shows  a  view 
of  a  transformer  mounted  on  an 
electric  pole  and  connected  to 
the  electric  light  wires  ready  for 
use.  As  the  transformer  is  ex- 
posed to  the  weather,  the  need 
of  the  protecting  case  is  made 
evident.  In  some  types  of  trans- 
formers it  is  usual  to  fill  up  the 
case  with  a  heavy  paraffine  oil, 
which  improves  the  insulation  of 
the  coils  from  each  other.  All 
transformers,  from  the  smallest 
one,  which  has  a  capacity  of  one 
or  two  lights,  up  to  several  hun- 
dred horse  power  capacity,  are 
very  similar  in  construction.  The 

various  manufacturers  make  differences  in  the  number  and  shape  of  the 
iron  plates  used  in  the  cores,  the  sizes  of  wires  and  numbers  of  turns 
composing  the  coils,  and  in  other  details,  but  the  greatest  differences 
apparent  to  the  sight  are  differences  in  the  shapes  of  the  iron  cases. 
But  though  the  real  differences  between  the  transformers  are  very  small, 
even  these  small  differences  affect  their  usefulness  very  much. 

240.  Transformer  Iron  Losses. — The  iron  core  of  a  transformer  is 
magnetized  first  in  one  direction  and  then  in  the  other  by  the  alternating 
currents  in  the  coils,  and  as  the  magnetic  molecules  are  reversed  there 


FlG.  168.  —  Transformer  on   Electric   Light 
Pole. 


ALTERNATING  CURRENTS  AND   MACHINERY  263 

is  a  loss  of  power  caused  by  hysteresis.1  There  is  also  a  loss  of  power 
caused  by  eddy  or  foucault  currents  2  which  are  set  up  in  the  iron  core. 
These  losses  are  quite  small  compared  with  the  full  load  of  the  trans- 
former (from  3  to  6  per  cent)  ;  but  when  a  great  many  lightly  loaded 
transformers  are  operated  all  day  long,  as  is  done  in  many  electric  light 
plants,  the  total  power  lost  may  cause  a  great  expense.  The  losses  in  the 
cores  of  transformers  should,  therefore,  always  be  tested  by  electric  light 
companies  before  the  transformers  are  put  into  service,  and  if  the  losses 
are  larger  than  they  ought  to  be,  the  transformers  should  be  sent  back 
to  the  makers.  The  tests  can  be  made  by  connecting  up  a  wattmeter  to 
a  transformer,  as  shown  in  Figure  166.  If  the  secondary  circuit  is  left 
open,  the  reading  of  the  wattmeter  shows  the  loss  of  power  caused  by 
hysteresis  and  foucault  currents.  The  following  table  shows  approxi- 
mately the  amount  of  power  which  is  lost  in  the  cores  of  transformers 
of  the  best  makes,  when  they  are  operated  by  alternating  currents  with 
a  frequency  of  sixty  periods  per  second  :  — 


CAPACITY  OF  TRANSFORMERS 


Loss  IN  CORE 


500  watts  =    10  lights 
1000     "       —    20      " 
2000     "       =    40      " 

3000       "  :      60         " 

5000     "      =  100      " 

10,000      "         =  200 


25  watts. 
35     " 

45     " 

55    " 

75     " 

125     « 


241.  Ratio  of  Transformation.  —  In  nearly  all  electric  lighting  plants 
where  alternating  currents  are  used  in  this  country,  the  pressure  gener- 
ated by  the  alternator  is  between  1000  and  1200  volts,  or  2000  and  2500 
volts,  while  the  pressure  desired  at  the  lamps  is  between  100  and  no 
volts,  or  50  and  55  volts.  The  transformer  coils  must  be  wound  so  that 
the  number  of  primary  turns  has  the  same  relation  to  the  number  of 
secondary  turns  as  the  primary  pressure  has  to  the  desired  secondary 

1  Article  202.  2  Article  201. 


264  ELECTRICITY   AND    MAGNETISM 

pressure.  If  the  pressure  is  reduced  from  1000  volts  to  100  volts,  there 
must  be  one-tenth  as  many  turns  in  the  secondary  winding  as  in  the 
primary,  and  if  the  pressure  is  reduced  to  50  volts,  the  secondary  wind- 
ing must  have  one-twentieth  as  many  turns  as  the  primary.  Since  the 
power  given  out  by  a  transformer  is  nearly  as  great  as  that  given  to  it, 
the  current  in  the  secondary  coil  is  nearly  as  many  times  greater  than 
the  primary  current  as  the  secondary  pressure  is  smaller  than  the  primary 
pressure. 

We  have  in  transformers  a  most  striking  and  wonderful  example  of 
the  transfer  of  power  from  one  electrical  circuit  (the  primary  circuit)  to 
another  circuit  (the  secondary  circuit)  without  the  circuits  being  in  any 
way  electrically  connected  with  each  other.  The  inductive  action  goes 
on  just  as  well  if  the  two  coils  of  the  transformer  are  separated  by  glass 
or  mica  as  if  they  are  wound  close  together.  //  is  only  necessary  for  the 
magnetic  circuit  to  be  properly  arranged  so  that  the  magnetism  which  is 
set  lip  by  the  primary  coil  shall  all  pass  through  tJie  secondary  coil.  The 
action  of  transformers  is  really  no  more  wonderful  than  the  action  of 
dynamos,  but  has  the  striking  peculiarity  that  no  mechanical  motion  is 
concerned  in  the  transformations.  In  both  machines  the  pressure  is 
caused  by  change  of  lines  of  force  in  the  generating  coils,  —  in  the 
dynamo  by  the  conductors  being  moved  through,  a  fixed  magnetic 
field,  and  in  the  transformer  by  setting  up  a  variable  magnetic  field 
through  the  fixed  secondary  coils. 

Example  A.  If  a  transformer  has  1000  turns  in  the  primary  winding  and  gives  a 
secondary  pressure  of  50  volts,  when  the  primary  pressure  is  1000  volts,  how  many 
turns  has  the  secondary  winding?  Ans.  50. 

Example  B.  A  transformer  receives  1000  volts  and  10  amperes  at  its  primary. 
If  the  secondary  pressure  is  100,  about  how  great  will  the  secondary  current  be? 
Ans.  100  amperes  (nearly). 

Example  C.  The  primary  and  secondary  of  a  transformer  respectively  consist  of 
2000  and  100  turns.  If  the  primary  receives  10  kilowatts  of  energy  at  2000  volts 
pressure,  what  will  be  the  current  (nearly)  and  the  pressure  of  the  secondary? 
Ans.  100  amperes  (nearly)  at  100  volts. 

242.  Alternators.  — As  already  said,  alternating  current  dynamos,  or 
alternators,  are  built  upon  the  same  principles  as  continuous  current 
dynamos,  but  the  armatures  are  commonly  wound  in  coils  which  are  con- 


ALTERNATING  CURRENTS  AND  MACHINERY 


265 


FIG.  169.  —  Connections  of 
Alternator  Armature. 


nected  in  series,  and  the  two  ends  are  brought  to  separate  collecting 

rings.     The  field  magnet  usually  has  as  many  poles  as  there  are  coils 

on  the  armature,  and  the  number  of  alterna- 

tions of  the  current  per  minute  is  equal  to  the 

number  of  poles  in  the  field  magnet  multiplied 

by  the  number  of  revolutions  made  by  the  arma- 

ture per  minute. 

Figure  169  shows  a  diagram  of  the  connec- 
tions of  an  alternator  armature.  The  coils 
marked  AA  are  armature  coils  and  the  rings 

i     j   ^x~,  ,i  11      .•  •  1-1 

marked  CC  are  the  collecting  rings  on  which 

the  brushes  BB  rub.     The  arrows  show  the 

way  the  current  flows  through  the  armature.  Figure  1  70  shows  the  way 
tRe  magnet  poles  are  arranged  for  an  alternator  having  an  armature  of 
the  form  shown  in  Figure  169.  This  arrange- 
ment of  the  armature  and  fields  was  formerly 
quite  common  in  foreign  alternators. 

In  this  country  the  coils  are  usually  laid  in 
grooves  cut  in  a  drum  armature  core.  Figure 
171  shows  the  way  in  which  coils  are  sometimes 
fixed  in  the  grooves. 

Since  no  commutator  is  required  with  an  alter- 

nator    it  js   not  necessary  for  the   armature   to 

' 

revolve,  and  the  field  may  be  revolved  instead. 
In  this  case,  the  magnetizing  current  is  carried  to  the  field  windings 
through  collector  rings,  and  the  armature  terminals  are  connected 


Fields  of  Al- 
ternator. 


FlG.  171.  —  The  Construction  of  an  Alternator  Armature. 


266 


ELECTRICITY  AND    MAGNETISM 


directly  to  the  circuit.  It  is  also  possible  to  build  alternators  in  which 
neither  the  field  nor  armature  revolves,  but  in  which  keepers  of  iron  are 
moved  so  as  to  make  and  break  the  magnetic  circuit  of  the  field  mag- 
nets and  thus  cause  currents  to  be  induced  in  the  stationary  armature. 
Such  machines  are  called  Inductor  Alternators. 

Example  A.  What  is  the  number  of  alternations  per  minute  set  up  by  an  alter- 
nator which  has  8  poles  and  the  armature  of  which  revolves  at  a  speed  of  1200  rev. 
per  minute?  Ans.  9600. 

Example  B.  What  is  the  frequency  of  an  alternator  which  has  10  poles  and  a 
speed  of  1000  rev.  per  minute?  Ans.  83^. 

Example  C.  At  what  speed  must  an  alternator,  having  8  poles,  run  to  give  a 
frequency  of  80  periods  per  second?  Ans.  1200  rev.  per  minute. 

243.  Field  Excitation.  —  The  field  magnets  of  an  alternator  must 
always  be  excited  by  a  continuous  current,  which  is  usually  generateo! 


EXCITER 


FIELD  WINDINGS 
ARMATURE 

COLLECTOR  RINGS 

"RECTIFYING  COMMUTATOR 


Fir..  172.  —  Alternator  and  Exciter. 

by  a  separate,  small,  continuous  current  dynamo  called  an  Exciter.     An 
exciter  is  shown  alongside  of  the  alternator  in  Figure  172.     When  an 


ALTERNATING   CURRENTS  AND   MACHINERY  267 

alternator  is  loaded,  the  pressure  at  its  terminals  will  decrease  in  a  way 
that  is  analogous  to  that  explained  with  reference  to  shunt-wound  direct 
current  dynamos.1  To  keep  the  pressure  at  proper  value,  therefore,  it 
is  necessary  to  have  a  hand  regulator  or  rheostat  in  either  the  field  of 
the  exciter  or  alternator,  or  both. 

The  pressure  may  be  kept  reasonably  constant,  without  much  hand 
regulating,  by  rectifying  a  portion  of  the  armature  current  by  means  of 
a  commutator  and  causing  it  to  pass  through  a  few  turns  of  wire  on  the 
field  magnet.  This  excitation  is  in  addition  to  that  furnished  by  the 
exciter.  Such  an  arrangement  is  called  Composite  excitation,  and  is 
similar  in  effect  to  compound  winding2  in  direct  current  machines. 
The  rectifying  commutator  is  shown  in  Figure  172,  just  in  front  of  the 
collector  rings. 

244.  Alternators   in    Parallel.  —  Alternators    cannot   be   worked   in 
parallel  with  each  other  with  the  ease  which  is  possible  with  continu- 
ous current  dynamos.     If  two  similar  shunt-wound  continuous  current 
dynamos  are  to  be  connected  in  parallel,  they  are  simply  brought  to 
their  usual  speeds,  and  their  field  magnetization  is  adjusted  until  the 
two  machines  produce  the  same  pressure.     They  may  then  be  con- 
nected in  parallel  and  will  work  together  very  well.     When  two  alter- 
nators are  to  be  connected  in  parallel,  it  is  necessary  not  only  to  make 
their  pressures  equal,  but  to  bring  them  to  exactly  equal  frequencies  or  to 
Synchronism,  and  also  to  arrange  them  so  that  the  current  loops  given  by 
the  two  machines  are  in  exact  unison  or  Step.     On  account  of  the  diffi- 
culty in   the  way  of  properly  Synchronizing  and  Stepping  alternators, 
they  are  not  usually  operated  in  parallel  in  this  country;  though  parallel 
operation  is  rapidly  becoming  less  uncommon. 

245.  Synchronous  Motors.  —  If  an  ordinary  alternator  is  brought  to 
synchronism  with  another  machine,  it  may  be  run  by  the  latter  as  a 
motor,  but  it  will  not  start  itself,  as  would  a  continuous  current  motor, 
nor  is  it  possible  to  excite  the  field  magnets  of  the  motor  from  the  alter- 
nating current  circuits.      It  is,  therefore,  not  convenient  to  use   such 
machines,  called  Synchronous  Motors,  for  common  purposes. 

Such  machines  are  not  self-starting,  because  the  rapidly  alternating 
currents  first  give  a  pull  in  one  direction  and  then  a  push  in  the  other. 
1  Article  210.  2  Article  210. 


268 


ELECTRICITY  AND    MAGNETISM 


After  the  machine  has  been  brought  up  to  synchronism,  however,  the 
coils  move  from  one  pole  piece  to  another  as  often  as  the  current  changes 
direction,  thus  making  a  tendency  to  turn  constantly  in  one  direction. 
Such  motors  have  been  arranged  to  be  started  by  a  small  steam  engine, 
a  storage  battery  driving  the  exciter,  or  other  very  cumbrous  means,  but 
in  recent  years  it  has  been  found  possible  to  build  small  alternating  cur- 
rent induction  motors  or  to  put  special  windings  upon  the  machines, 
which  perform  the  starting  duty.  Polyphase  machines,  which  will  be 
dealt  with  in  the  next  article,  have  done  much  toward  making  such 
motors  available. 

246.  Polyphase  Currents.  —  A  second  set  of  windings  may  be 
placed  on  an  alternator  armature,  with  the  centres  of  its  coils  halfway 
between  the  first  set  (as,  for  instance,  if  another  winding  were  put  on 
the  armature  shown  in  diagram  in  Figure  169,  with  its  coils  between 
those  shown  in  the  figure) ;  then  the  currents  generated  in  the  second 

set  of  coils  have  their  maximum  points 
just  one- quarter  of  a  period  after  the 
currents  in  the  first  winding.  That  is, 
the  two  currents  have  a  difference  of 
phase  equal  to  quarter  of  a  period,  or 
90  degrees. 

The  relation  of  these  two  currents  to 
each  other  is  shown  in  Figure  1 73,  where 
the  curves  A  and  B  represent  the  two 

current  waves.  These  two  currents  may  be  used  separately,  or  they 
may  be  used  together  as  a  Two-phase  sys- 
tem with  the  two  currents  carried  in  cir- 
cuits composed  of  three  wires  very  much  as 
the  three  wires  compose  the  circuits  of  the 
three-wire  system  for  continuous  current 
distribution,  which  is  described  in  a  later 
chapter. 

Instead  of  two  windings,  three  separate 
windings  may  be   placed  on  the   armature 

in  such  a  way  that  the  three  currents  produced  in  them  differ  from 
each  other  in  phase  by  one-third  of  a  period,  or  120°.  The  relations 


FIG.  173.  —  Diagram  of  Currents 
in  Two-phase  System. 


FIG.    174.  —  Diagram    of  Cur- 
rents in  Three-phase  System. 


ALTERNATING  CURRENTS  AND  MACHINERY 


269 


FIG.  175. —  Diagram 
of  Circuits  for  the 
Individual  Cur- 
rents of  Three- 
phase  System. 


of  these  currents  are  illustrated  in  Figure   174,  where  the   curves  A, 

B,  and  C  represent  the  three  current  waves.     These  currents    may  be 

used  separately  or  they  may  be  used  together  as  a 

Three-phase  system   with   the   three   currents   carried 

in  circuits  composed  of  three  wires.      In  this   case, 

if  the   three  dots  marked  a,  b,  and  ct  in  Figure  175 

represent  the  cross  sections  of  the  three  wires,  then 

current  A  is  carried  in  the  circuit  composed  of  the 

wires  a  and  b,  current   B  is  carried  in  the  circuit 

composed   of  the   wires  b  and  c,   and   current  C  is 

carried  in  the  circuit  composed  of  the  wires  c  and  a. 

Either  a  two-phase  or  three-phase  alternator  which 
•is  arranged  to  furnish  currents  to  three  wires  only,  requires  three  collect- 
ing rings,  though  if  the  currents  are  to  be  used  separately,  four  and  six 
rings  may  be  used.  Two-phase  and  three-phase  systems  are  frequently 
called  Polyphase  or  Multiphase  (many-current)  systems,  and  the  motors 
which  are  ordinarily  operated  on  polyphase  systems  are  called  Induction 
Motors. 

Polyphase  alternators  may  be  used  as  synchronous  polyphase  motors 
under  conditions  similar  to  those  already  explained  for  single-phase 
machines. 

247.  Induction  Motors.  — The  action  of  induction  motors  may  be  ex- 
plained by  reference  to  Figure  176,  which  is  an  illustrative  diagram  of  a 
three-phase  motor.  The  field  magnet  of  the 
motor  is  a  ring  which  is  wound  with  three  sep- 
arate coils,  X,  Y,  and  Z,  each  of  which  is  sup- 
plied with  one  of  the  currents  of  the  three-phase 
system  through  the  wires  a,  b,  and  c.  Since  the 
maximum  values  of  the  three  currents  which 
thus  flow  through  the  coils  X,  Y,  and  Z,  follow 
one  another  with  a  phase  difference  of  a  third  of 
a  period,  their  maximum  points  appear  to  chase 
each  other  around  the  ring.  The  magnetic 
FIG.  176.  —  illustrative  Di-  effect  of  each  coil  at  every  instant  is  propor- 

agram  of  a  Three-phase     tional  to  the  current  flowing  in  it,  and  the  corn- 
Motor. 

bmed  effect  of  the   three   currents    sets   up   a 


2/0 


ELECTRICITY  AND   MAGNETISM 


ARMATURE  CORE 


FIG.  177.  —  Armature  of  Induction  Motor. 


magnetic  field  which  rotates  around  the  ring  along  with  the  maximum 
values  of  the  currents. 

The  space  inside  of  the  field  ring  is  occupied  by  an  armature  which 
consists  of  a  grooved  drum  made  of  iron  disks.  Insulated  copper  rods 
are  hid  in  the  grooves,  and  the  rods  are  all  connected  together  by  end 

rings,  as  in  Figure  177,  or  they 
are  connected  in  sets,  as  indi- 
cated by  the  dotted  lines  in 
Figure  176. 

The  rotating  magnetic  field 
set  up  by  the  currents  in  the 
windings  X,  Y,  and  Z,  induces 
currents  in  the  armature  con- 
ductors, and  these  in  turn,  on 
account  of  the  reactions  be- 
tween currents  and  a  magnetic  field  explained  in  Article  207,  cause  the 
armature  to  revolve  nearly  in  synchronism  with  the  rotating  field.  This 
motor  speed  is  not  quite  in  synchronism,  however,  as  otherwise  no  cur- 
rent would  be  generated  to  react  upon  the  field. 

The  armature,  with  the  bars  all  connected  together,  as  shown  in  Figure 
177,  is  called  a  Squirrel-cage  Armature. 

248.  Rotary  Converters.  —  It  will  be  remembered  that  the  current  in  the 
armature  coils  of  direct  current  machines  is  alternating  and  must  be  corn- 
mutated  if  direct  currents  are  to  be  generated.1  Therefore,  if  connections 
are  made  to  the  opposite  sides  of  the  armature 
of  a  bipolar  dynamo  and  are  carried  to  collect- 
ing rings,  alternating  currents  may  be  drawn 
off  (see  Fig.  178).  Such  a  machine  will  gen- 
erate both  direct  and  alternating  currents.  If 
the  machine  is  run  as  a  direct  current  motor, 
alternating  currents  may  be  drawn  from  the 
collector  rings,  and  in  this  manner  direct  cur- 

Fio.  178.  —  Rotary  Converter. 

rents  may  be  transformed  into  alternating  cur- 
rents.    Likewise,  if  the  machine   is  run  as  a  synchronous  alternating 
current   motor,  direct  currents  may  be  taken  from  the  commutator. 

1  Article  197. 


WIRES  TO  RINGS 
j    j     /Qv          COMMUTATOR) 


ALTERNATING  CURRENTS  AND   MACHINERY  2/1 

Thus  are  alternating  currents  converted  into  direct  currents.  If  three- 
phase  alternating  currents  are  desired,  three  rings  are  connected  to 
three  points,  120°  apart,  and  if  two-phase  alternating  currents  are  de- 
sired, four  connections  90°  apart  are  used. 

In  the  case  of  multipolar  dynamos,  a  set  of  connections  must  be  taken 
to  the  rings  for  each  pair  of  poles.  . 

Much  of  the  power  from  the  great  Niagara  power  plant  is  transformed 
by  rotary  converters  into  direct  currents. 

QUESTIONS 

48.  What  effect  has  current  lag  upon  the  power  in  an  alternating  current  circuit  ? 

49.  Why  does  the  product  of  effective  current  and   pressure  in  an  alternating 
current  circuit  not  always  give  the  power  ? 

50.  Will  an   effective   current  of   10  amperes  under  an  effective   pressure  of  20 
volts  usually  give  the  same  power  as  a  continuous  current  of  10  amperes  under  a 
pressure  of  20  volts  ?     Why  not  ? 

51.  Would  any  power  be  given  out  in  a  circuit  if  the  current  and  pressure  were 
90°  apart  ? 

52.  Can  a  current  actually  have  a  lag  of  90°  behind  the  pressure  that  produces  it? 

53.  Describe  the  power  loops  in  an  inductive  alternating  current  circuit. 

54.  What   pressures  combine   to   furnish  the  pressure  which  drives  the  current 
through  an  alternating  current  circuit  ? 

55.  How  can  a  voltmeter  and  amperemeter  be  used  for  measuring  the  power  given 
by  an  alternating  current  to  an  incandescent  lamp  ? 

56.  \Vhy  can  the  amperemeter  and  voltmeter  be  used  for  the  purpose  described 

^  55? 

57.  Why  cannot  an  amperemeter  and  voltmeter  be  used  for  measuring  the  power 
in  a  self-inductive  circuit  ? 

58.  WThy  will  a  wattmeter  give   the   true  power  in  either  an   inductive  or  non- 
inductive  circuit? 

59.  What  are  transformers? 

60.  How  are  transformers  made? 

61.  What  advantage  do  transformers  lend  for  transmitting  power? 

62.  Why  are  the  iron  losses  in  transformers  very  serious  to  lighting  companies? 

63.  How  can  transformers  be  tested  to  find  the  amount  of  iron  losses? 

64.  What  relations  have  the  pressures,  turns,  and  currents  in  the  primary  and  sec- 
ondary coils  of  a  transformer? 

65.  Compare  a  transformer  with  a  dynamo. 

66.  What  is  the  difference  between  an  alternator  and  a  direct  current  dynamo? 

67.  How  can  the  frequency  of  the  current  from  an  alternator  be  determined? 


2/2  ELECTRICITY   AND    MAGNETISM 

68.  What  is  an  inductor  alternator? 

69.  How  are  the  Held  magnets  of  alternators  excited? 

70.  What  is  composite  winding? 

71.  What  precautions  must  be   observed  before   alternators  can  be  thrown  in 
parallel?     Why? 

72.  What  is  meant  by  saying  that  two  alternators  are  in  synchronism? 

73.  What  is  a  synchronous  motor? 

74.  Why  are  synchronous  motors  not  self-starting? 

75.  Explain  the  words  "  polyphase  "  and  "  multiphase." 

76.  What  are  two-  and  three-phase  alternators? 

77.  Describe  a  two-phase  system  of  currents. 

78.  Describe  a  three-phase  system. 

79.  What  makes  the  magnetic  field  rotate  in  an  induction  motor? 

80.  Describe  a  squirrel-cage  armature. 

81.  Why  does  the  armature  of  an  induction  motor  rotate? 

82.  How  is  the  current  in  the  armature  of  an  induction  motor  produced? 

83.  What  are  rotary  converters? 

84.  How  are  rotary  converters  made?     How  do  they  work? 


CHAPTER  XVII 

ARC   AND   INCANDESCENT  LIGHTING 

249.  The  Electric  Arc.  — The  Arc  Lights  which  are  so  much  a  neces- 
sity to-day  for  illuminating  the  streets  of  cities  and  all  large  spaces  which 
require  a  high  degree  of  illumination,  whether  indoors  or  out,  are  the 
direct  commercial  outgrowth  of  a  magnificent  discovery  which  was  an- 
nounced shortly  after  1800.  This  discovery  was,  indeed,  nothing  less 
than  the  possibility  of  producing  the  common  Electric  Arc.  The  dis- 
coverer of  the  electric  arc,  Sir  Humphry  Davy,  the  great  English 
scientist,  exhibited  it  on  a  grand  scale  in  1808  in  a  lecture  before  the 
Royal  Institution  in  London,  when  he  connected  the  electric  circuit 
from  a  battery  of  two  thousand  or  more  cells  through  two  pieces  of 
charcoal  and  then  gradually  separated  them.  The  result  was  an  arch  or 
"  arc  "  of  dazzling  light  between  the  charcoal  tips  such  as  had  never 
before  been  artificially  produced.  Sir  Humphry  Davy's  experiments 
created  a  great  deal  of  interest,  but  the  real  usefulness  of  the  electric  arc 
was  not  seen  until  Faraday's  later  discoveries  had  laid  the  foundation 
for  the  development  of  the  dynamo  and  the  economical  production  of 
electricity. 

The  means  for  producing  this  arc  of  light  are  comparatively  simple. 
When  two  pointed  pieces  of  carbon  (made  from  charcoal,  coke,  etc.) 
are  joined  to  opposite  poles  of  the  circuit  from  a  powerful  generator  of 
electricity  and  are  touched  together,  a  current  flows  between  them. 
A  considerable  resistance  exists  where  their  points  are  in  contact,  and 
the  points  are  heated  by  the  current  unless  they  are  pressed  very  tightly 
together.  If  the  contact  is  quite  loose,  the  points  become  so  hot  as  to 
cause  the  carbon  to. pass  off  as  vapor.  Now,  if  the  carbon  points  are 
separated,  the  current  continues  to  flow  across  the  space  between  the 
points,  which  is  filled  with  carbon  vapor,  forming  the  electric  arc. 
Carbon  vapor  is  a  much  better  conductor  of  electricity  than  air,  and 
the  current  can,  therefore,  be  caused  to  flow  across  a  space  filled  with 
T  273 


274 


ELECTRICITY   AND    MAGNETISM 


it,  though  it  could  not  readily  be  caused  to  flow  continuously  through 
the  same  space  filled  with  air. 

It  seems  strange  to  speak  of  the  vapor  of  carbon,  but  the  temperature 
of  the  electric  arc  is  so  great  that  it  boils  and  vaporizes  the  most  refrac- 
tory materials.  The  vaporizing  of  any  material  is  merely  a  question  of 
temperature,  and  the  vaporization  of  carbon,  platinum,  gold,  iron,  cop- 
per, etc.,  in  the  electric  arc  is  just  as  simple  as  the  conversion  of  water 

into  steam  (the 
vaporization  of 
water)  over  a  com- 
mon coal  fire.  The 
vaporization  of 
"  refractory  "  ma- 
terials like  carbon, 
platinum,  et  cete- 
ra, simply  requires 
a  much  higher  tem- 
perature than  that 
which  is  reached 
by  the  coal  fire 
that  is  amply  suffi- 
cient to  boil  water. 
After  a  direct- 
current  arc  has 
existed  for  a  little 

time  between  the 
FIG.  179. —  The  Electric  Arc.  .    L      , 

carbon  points,  they 

come  to  look  very  much  as  they  are  shown  in  Figure  1 79.  Both  points 
become  quite  hot  and  give  off  light,  but  the  positive  point  (which  is  the 
upper  point  in  the  figure)  becomes  much  hotter  than  the  negative,  and 
from  it  comes  the  greater  part  of  the  light  of  the  arc.  In  an  arc  which 
is  set  up  with  a  continuous  current,  carbon  is  carried  off  by  the  current 
from  the  positive  point  but  not  from  the  negative  point.  The  positive 
point,  therefore,  becomes  a  little  hollowed  out  on  the  end  as  shown 
in  the  figure.  This  hollow  is  called  the  Crater  of  the  arc. 

As  the  greater  part  of  the  light  of  the  direct-current  arc  comes  from 


ARC   AND   INCANDESCENT   LIGHTING 


275 


this  positive  end  or  crater,  the  positive  carbon 
in  an  arc  lamp  is  almost  always  put  at  the 
top,  in  order  that  the  light  may  be  thrown 
downward.  When  an  arc  is  set  up  with  an 
alternating  current,  both  points  become  some- 
what crater-like  an4  light  is  given  off  about 
equally  from  the  two  points. 

Since  the  arc  is  surrounded  by  air,  the  car- 
bon of  which  the  points  are  composed  is  grad- 
ually burned  up,  and  if  the  carbons  are  fixed 
in  position,  the  arc  grows  longer  and  longer 
until  its  resistance  becomes  so  great  that  the 
current  cannot  pass  through  it ;  the  current 
then  stops  and  the  arc  goes  out.  Since  car- 
bon is  carried  away  from  the  positive  point, 
but  not  from  the  negative  point,  the  former 
wastes  away  in  a  direct-current  arc  at  a  rate 
which  is  just  about  double  that  of  the  latter. 
The  two  carbons 
of  an  alternating- 
current  arc  waste  away  at  approximately  equal 
rates.  The  expenditure  of  carbon  grows 
larger  with  the  current  and  is  approximately 
independent  of  the  size  of  the  carbons,  so 
that  carbons  of  large  diameters  have  a 
"longer  life"  than  smaller  carbons. 

250.  Arc-lamp  Mechanism.  —  In  order  that 
the  electric  lamp  may  be  used  for  commercial 
lighting,  an  automatic  device  must  be  used  to 
keep  the  carbons  fed  toward  each  other  as 
they  waste  away,  so  that  the  arc  shall  always 
have  the  proper  length.  This  is  included  in 
the  mechanism  of  what  is  known  as  an  Arc 
Lamp.  This  consists  of  a  case  which  contains 
the  feeding  mechanism,  below  which  is  a 
frame  to  support  the  lower  or  negative  carbon  and  a  glass  shade. 


FIG.  179  a.— The  Electric  Arc 
produced  in  an  Open  Arc 
Lamp  by  a  Direct  Current. 


FIG.  179/5.  — The  Electric 
Arc  produced  in  an  Open 
Arc  Lamp  by  an  Alternat- 
ing Current. 


ELECTRICITY   AND   MAGNETISM 


The  feeding  mechanism  has  two  duties  to  perform  :  — 

1.  To  separate  the  carbons,  or  strike  the  arc,  when  the  lamp  is  thrown 
into  circuit. 

2.  To  regulate  the  movement  of  the  upper  or  positive  carbon  down- 
ward toward  the  negative  one  as  the  carbons  wear  away. 

The  lower  carbon  is  usually  clamped  solidly  at  the  bottom  of  the 
lamp  frame,  and  the  upper  one  is  clamped  at  the  end  of  a  polished 
brass  Carbon  Rod,  the  motion  of  which  is  controlled  by  the  mechanism. 


FIG.  180.  —  Diagram  of  Clock-work  Arc- 
lamp  Mechanism. 


FIG.    181. —  Diagram   of  Clutch  Arc- 
lamp  Mechanism. 


Figures  180  and  181  show  familiar  forms  of  arc  lamps,  intended  for 
use  in  series  circuits.  The  mechanism  of  such  lamps  is  usually  caused 
to  operate  by  the  opposing  action  of  two  electromagnets,  or  an  electro- 
magnet and  a  spring.  The  windings  of  one  of  these  are  composed  of  a 
few  turns  of  comparatively  coarse  wire  which  are  connected  directly  into 
the  circuit  in  series  with  the  arc.  The  windings  of  the  other  magnet  are 


ARC   AND    INCANDESCENT   LIGHTING 


2/7 


made  of  many  turns  of  comparatively  fine  wire  which  are  connected  as  a 
shunt  to  the  arc.  The  two  electromagnets  may  be  plainly  seen  in  Figure 
1 80.  In  some  lamps,  both  windings  are  put  on  the  same  magnet. 

The  purpose  of  the  windings  is  the  same  in  the  two  arrangements,  and 
may  be  explained  by  reference  to  Figure  180.  A  brass  lever  which  runs 
across  the  lamp  carries  an  iron  armature  or  plunger  at  one  end.  The 
armature  is  in  such  a  position  that  it  is  attracted  by  one  of  the  two  elec- 
tromagnets, and  the  lever  is  attached  to  the  mechanism  which  controls 
the  carbon  rod.  The  lamp  is  Trimmed  or  Carbonned  with  the  tips  of  the 
two  carbons  resting  against  each  other.  When  the  lamp  is  thrown  into 
circuit,  the  full  current  of  the  circuit  flows  through 
the  Series  Winding,  and  the  lever  is  lifted  by  the 
attraction  of  the  Series  Magnet.  This  causes  the  mech- 
anism to  raise  the  carbon  rod  sufficiently  to  Strike 
the  arc.  As  the  carbon  burns  away,  the  electric 
pressure  between  their  points  becomes  greater,  so 
that  the  current  in  the  Shunt  Coil  increases.  The 
armature  of  the  shunt  coil  is  attracted  more  and  more 
strongly,  until  the  clutch  or  pawl  releases  the  carbon 
rod  sufficiently  for  it  to  slide  slowly  downward,  and 
thus  Feed  the  positive  carbon  toward  the  negative. 

In  order  that  the  lamp  may  burn  smoothly  and 
quietly  it  is  necessary  for  the  feeding  mechanism  to 
keep  the  carbons  at  a  uniform  distance  apart  while 
the  lamp  is  burning.  This  can  only  be  accomplished 
when  the  magnetizing  coils  are  properly  balanced 
against  each  other,  and  the  strength  of  the  spring, 
which  acts  on  the  lever,  is  properly  adjusted.  Even 
when  all  the  adjustments  are  exactly  right,  arc  lamps 

will  not  burn  well  unless  the  carbons  are  of  uniform  FIG.  182.  —  Enclosed 
v,  i  V   •  Arc     Lamp     with 

quality.     In  some  arc  lamps  the  carbon  rod  is  con-      Cover  removed< 

trolled  by  a  clock-work,  which  in  turn  is  controlled 

by  the  Differential  Magnets  (Fig.  180),  while  in  others  a  simple  clutch 

is  caused  to  act  on  the  carbon  rod  by  the  magnets  (Fig.  181). 

In  another  style  of  lamp  the  differential  action  of  the  magnets  is  not 
utilized,  but  the  pull  of  the  shunt  magnet  is  arranged  to  act  against  the 


278 


ELECTRICITY   AND    MAGNETISM 


force  of  a  spring.     This  style  of  lamp  is  trimmed  so  that  a  little  space 
remains  between  the  carbon  points. 

251.  Enclosed  Arc  Lamps.  — It  has  been  found  very  recently,  about 
1895-1896,  that  an  arc  can  be  successfully  maintained  in  a  fairly  tight 
globe  ;  and  that  by  so  excluding  the  air  a  pair  of  ordinary  carbons  can 
be  made  to  burn  from  60  to  125  hours.  The  Enclosed  lamp  (Fig.  182) 
also  burns  with  a  steadier  light  than  the  Open  Arc,  so  that  it  is  more 

desirable  for  indoor  lighting. 

This  lamp  uses  about  the  same  power  per 
candle  power,  but  about  twice  the  pressure 
and  one-half  the  current  required  by  the  open 
arc.  The  lamps  are  largely  used  on  constant 
pressure  systems  though  they  are  coming  into 
use  for  series  street  lighting.  The  mechanism 
may  be  like  that  described  above,  but  series 
coils  alone,  acting  against  a  spring,  instead  of 
differential  coils,  are  generally  used.  The 
enclosing  globe  must  not  be  absolutely  air 
tight  or  it  will  explode,  and  indeed  it  would 
be  difficult  to  make  it  air  tight,  since  the 
upper  carbon  must  be  fed  through  an  open- 
ing in  the  top  plate. 

252.  Candle  Power  and  Operation  of  Arc 
Lamps.  — As  a  general  rule,  arc  lamps  are 
connected  in  series l  so  that  the  same  current 
passes  through  all.  This  current  is  usually 
furnished  by  a  series  dynamo  which  auto- 
matically keeps  the  magnitude  of  the  cur- 
rant constant.  The  constancy  of  the  current  is  a  very  important 
element  in  the  proper  regulation  of  the  lamps.  Nearly  all  open  arc 
lamps  are  now  adjusted  so  that  ih^  pressure  required  to  pass  the  current 
through  the  arc  is  from  45  to  50  volts.  If  the  pressure  is  made  smaller, 
the  arc  becomes  shorter  and  gives  less  light,  and  it  produces  a  continu- 
ous hissing  or  frying  sound.  If  the  pressure  is  greater,  the  arc  Flames 
and  flickers,  which  makes  it  unsatisfactory.  The  current  used  usually 
approximates  9.6,  6.5,  or  4  amperes. 

1  Article  102. 


FlG.  18211. —  Electric  Arc  pro- 
duced in  an  Enclosed  Lamp 
by  a  Direct  Current. 


ARC   AND   INCANDESCENT   LIGHTING 


2/9 


Arc  lamps  which  are  intended  to  be  used  with  9.6  amperes  are  usually 

spoken  of  as  2000  nominal  candle  power  or  450  watt  lamps,  while  those 

intended  to  be  used  with  6.5  and  4  amperes 

are  usually  called  1200  nominal  candle-power 

and  600  nominal  candle-power  lamps. 

A  candle  power  is  equal  to  the  light  given 

off  by  a  sperm  candle  of  fixed  size  and  form. 

The  actual  useful  candle  power  given  off  by 

the  lamps  is  much  less  than  the  figures  given 

in  the  last  paragraph,  and  in  fact  the  light 

given  off  in  different  directions  varies  from 

a  hundred  candle  power  or  thereabouts  to 

nearly  the  rated  value  of  the  lamp.     Figure 

183  shows,  by  the  curve,  the  amount  of  light 

given  off  by  arc  lights  in  different  directions 

when  using  various  currents. 

The  greatest  amount  of  light  is  given  off 

from  direct-current   open   arc    lamps  at  an 

angle  of  about  45°  from  the  direction  of  the 

carbons.     For  this  reason  the  best  effect  may 

be  gained  from  arc  lights  used  in  illuminat- 
ing streets  by  hanging  them  from  25  to  35 

feet  from   the  ground   over   the  centres  of 

streets,  or  by  mounting  them  at  street  corners  on  tall  poles  such  as  that 
shown  in  Figure  184.  Inside  of  buildings 
they  are  usually  hung  from  small  boards  fast- 
ened to  the  ceiling.  An  enclosed  switch  is 
usually  placed  in  arc  lighting  wires  where  they 
enter  a  building. 

As  the  carbons  which  are  ordinarily  used  in 
open  arc  lamps  are  of  such  a  length  that  they 
will  only  burn  for  seven  or  eight  hours,  Double 
Lamps,  which  have  two  sets  of  mechanism  and 
two  carbon  rods,  as  shown  in  Figure  184,  are 
used  for  all-night  lighting.  These  consist  of  a 

modified  mechanism  which  controls  two  carbon  rods,   one  of  which 


FIG.  182*.  —  Electric  Arc  pro- 
duced in  an  Enclosed  Lamp 
by  an  Alternating  Current. 


FIG.  183. —Candle-power 
Curves. 


280 


ELECTRICITY    AND    MAGNETISM 


does   not   come  into   service  until   the  carbons  held  in  the  first  have 
burned  out. 

The  carbons  that  are  ordinarily  used  vary  from  |-  to  \ 
inch  in  diameter,  and  are  usually  coated  with  copper  to 
reduce  their  resistance.  The  positive  carbon  is  about 
12  inches  long  and  the  negative  is  about  6  inches  long. 
The  carbons  are  made  from  finely  ground  coke  or  lamp- 
black which  is  mixed  with  syrupy  compounds  and  then 
baked  in  moulds,  or  by  some  equivalent  process.  The 
copper  coating  is  put  on  by  electroplating.  Oval  car- 
bons about  i  inch  broad  and  -J  inch  thick  have  been 
used  in  single  lamps  for  all-night  burning.  • 

253.  Arc  Machines  and  Switchboards.  —  The  number 
of  successful  manufacturers  of  series  arc-lighting  machin- 
ery is  comparatively  small.  The  earliest  to  enter  the 
business  in  this  country,  with  commercial  success,  was 
the  Brush  Electric  Company,  and  to  this  company  is 
probably  due  the  introduction  of  lamps  with  differential 
magnets,  which  are  still  so  much  used.  The  Brush  arc- 
light  dynamo  is  shown  in  Figure  185.  Figures  186  and 
187  show  other  types  of  arc-light  dynamos.  The  regula- 
tion of  these  is  performed  by  moving  the  brushes  around 
the  commutator,  or  shunting  the  field  windings,  so  that 
the  pressure  is  varied  as  lamps  are  cut  into  and  out  of 
circuit,  and  the  current  is  thus  always  kept  of  constant 
value. 

In  order  that  the  dynamos  in  a  series  arc-light  gener- 
ating station  may  be  properly  managed,  it  is  necessary 
to  have  some  arrangement  by  which  any  dynamo  in  the 
station   may  be    connected   to  any  one   of  the   circuits 
which  run  out  to  the  lamps.     The  number  of  dynamos 
—  Street  anc^  circmts    may  be   quite    large    in   a  plant  which   is 
Lamp-post   for  located  in    a   large    city,  and   the  arrangement  that   is 
Arc  Light.         usually  used  for  the  purpose  is  a  switchboard  (Fig.  188) 
fitted   with   a  heavy  spring  jack   for  each  wire    leading   to    the   lamp 
circuits.     The  spring  jacks  may  be   connected   as   desired   by  plugs 


ARC  AND   INCANDESCENT   LIGHTING 


281 


FIG.  185.  —  Brush  Arc-light  Dynamo. 


and  cords.  The  figure  shows  a  switchboard  arranged  for  three  lamp 
circuits  marked  i,  2,  3,  and  for  three  dynamo  circuits  marked  A, 
B,  C.  Each  dynamo  is 
shown  to  be  connected  to 
a  lamp  circuit  by  means 
of  plugs  and  cords.  The 
amperemeters  at  the  top 
of  the  switchboard  are 
connected  in  the  dynamo 
circuits  and  serve  to  show 
the  dynamo  attendant 
whether  or  not  the  ma- 
chines are  regulating  prop- 
erly. 

254.   Incandescent  Light- 
ing. —  Illumination  by  arc 
lights  is  very  satisfactory  in 
streets  or  open  spaces  out  of  doors  or  in  large  rooms  such  as  shops 
or  halls,  but  its  intense  brilliancy  causes  it  to  cast  dense  shadows  which 

totally  unfit  it  for  sat- 
isfactory use  in  gen- 
eral indoor  lighting. 
Its  unavoidable  flick- 
ering and  occasional 
hissing  also  make  it 
unsatisfactory  for  gen- 
eral use  in  small 
rooms.  If  the  faults 
of  the  arc  when  used 
for  general  indoor 
lighting  were  not  so 
evident,  the  use  of 
small  arcs  in  office 
and  house  lighting 
might  have  been  at- 

FIG.  186.  — Thomson-Houston  Arc-light  Dynamo.  tempted     as    early    as 


282 


ELECTRICITY   AND    MAGNETISM 


1880,  by  which   time   the  arc  lamp  had 
outdoor  lighting.     The  disadvantages  of 


FIG.  187.  —  Wood  Arc-light  Dynamo. 

by  means  of  a  current.  The  light  was, 
of  the  great  heat  caused  in  the  wire 
when  a  current  flowed  through  the 
high  resistance  of  the  wire.1  This  is  a 
case  where  the  C'1R  loss  was  turned  to 
a  useful  account,  but  the  lamps  were 
not  successful,  though  the  same  princi- 
ple is  used  in  the  incandescent  lamps 
of  to-day. 

Just  previous  to  1880  many  promi- 
nent inventors,  including  Edison, 
Maxim.  Farmer,  Sawyer,  and  Man  in 
this  country,  and  Swan  in  England, 
were  making  every  effort  to  construct 
a  satisfactory  lamp  to  operate  by  the 
incandescence  of  some  material.  It 
was  found  that  loops  of  platinum  and 

1  Article  112. 


begun  to  prove  its  value  for 
the  arc  for  general  illumina- 
tion had  become  known 
by  that  time,  and  invent- 
ors were  using  every 
effort  to  find  some  sub- 
stitute. 

Many  years  earlier, 
inventors  had  made 
electric  lamps  which 
consisted  of  a  loop  of 
wire  made  of  platinum 
or  iridium,  two  metals 
which  melt  only  at  ex- 
ceedingly high  tempera- 
tures, and  in  which  the 
light  was  produced  by 
heating  the  wire  white 
hot,  or  to  Incandescence, 
therefore,  produced  by  means 


tchboard. 


ARC  AND   INCANDESCENT   LIGHTING  283 

iridium  were  unsatisfactory  because  they  soon  melted  or  gave  out  when 
continuously  subjected  to  the  high  temperature  which  is  necessary  to 
produce  a  satisfactory  light.  The  only  conducting  material  which  would 
stand  the  high  temperature  of  incandescence  was  found  to  be  carbon. 
Unfortunately  carbon  burns  away  when  heated  to  a  high  temperature  in 
the  air,  and,  therefore,  could  not  be  used  in  a  lamp  in  the  same  way 
that  metallic  wires  had  been. 

255.  The  Carbon  Filament  Lamp.  —  As  early  as  1845  a  ^amP  na-d  been 
made  in  which  a  thin  stick  of  carbon  was  enclosed 

in  a  glass  globe  from  which  the  air  had  been  ex- 
hausted. This  lamp  produced  an  excellent  light, 
as  the  carbon  could  not  burn  away  in  a  vacuum, 
however  hot  it  became,  but  no  satisfactory  arrange- 
ments then  existed  for  making  proper  carbon  sticks 
or  for  exhausting  the  air  from  the  glass  globes. 
Shortly  before  1880  the  inventors  turned  from  their 
efforts  to  make  a  satisfactory  loop  from  a  metal 
wire,  to  make  another  attempt  to  use  carbon.  By 
1880  Edison,  Sawyer  and  Man,  and  Swan  had 
made  lamps  which  produced  light  through  the 

incandescence  of  a  thin  strip  or  Filament  of  car-  FIG-  189.  —  Early  Edison 

Incandescent  Lamp, 
bon. 

The  lamp  made  by  Edison  looked  very  much  like  the  incandescent 
electric  lamps  of  the  present  day,  and  it  is  no  doubt  to  his  industry  and 
ingenuity  that  we  owe  the  introduction  of  the  cheap  and  economical 
form  of  incandescent  lamp  which  we  now  use.  One  of  Edison's  early 
lamps  is  shown  in  Figure  189.  The  globe  or  Bulb  of  the  lamp  contained 
a  filament  of  carbonized  paper  in  an  arched  or  horseshoe  form.  The 
ends  of  the  carbon  horseshoe  were  connected  to  short  pieces  of  platinum 
wire  which  passed  through  the  glass  of  the  bulb.  By  means  of  these 
wires  a  current  could  be  led  to  the  filament.  The  bulb  was  Exhausted 
(that  is,  the  air  was  removed)  by  means  of  a  form  of  mercury  air-pump, 
which  is  used  in  a  modified  form  for  the  same  purpose  at  the  present 
day,  and  which  is  capable  of  producing  a  very  perfect  vacuum. 

256.  Exhausting  Incandescent  Lamps.  —  Figures  190  and  191  show 
the  two  forms  of  air-pumps  which  have  been  commonly  used  in  exhaust- 


284 


ELECTRICITY   AND    MAGNETISM 


ing  lamps.  These  are  often  called  vacuum  pumps  because  they  are 
used  to  produce  a  vacuum.  The  first  is  called  the  Geissler  Pump  after 
its  inventor,  who  was  also  the  maker  of  the  vacuum  tubes  known  as 
Geissler  tubes,  which  display  such  pretty  color 
effects  when  an  electric  spark  is  passed  through 
them.  The  pump  shown  in  the  second  figure  is 
called  a  Sprengel  Pump,  also  after  the  name  of 
its  inventor. 

The  operation  of  the  Sprengel  pump  (Fig.  191) 
is  quite  similar  in  principle  to  the  operation  of 
some  injectors.  The  mercury  is  allowed  to  flow 
in  a  jet  through  the  nozzle  J,  and  air  is  drawn 
from  the  lamps  by  the  suction  of  the  drops  of 
mercury  rushing  past  the  end 
of  the  lamp  tap.  The  Geiss- 
ler pump  is  more  complicated 
in  its  action,  but  briefly,  the 
bulb  B2  is  filled  with  mercury, 
which  is  then  drawn  out,  leav- 
ing a  vacuum,  which  in  turn  draws  air  from  the 
lamp.  This  process  is  repeated  again  and  again 
until  a  satisfactory  degree  of  exhaustion  is  pro- 
duced. 

257.  The  Production  of  Carbon  Filaments. —The 
carbon  filaments  of  incandescent  lamps  are  fre- 
quently made  from  bamboo  strips  or  from  silk  or 
cotton  threads,  though  recently  they  are  more  usu- 
ally made  by  squirting  a  glutinous  substance  through 
a  small  aperture  and  hardening  it  in  a  vessel  of 
water.  These  are  converted  into  carbon  by  baking, 
in  very  much  the  same  way  that  wood  is  converted  into  charcoal  in 
a  kiln.  The  material  is  first  made  into  exactly  the  proper  size  to  pro- 
duce a  filament.  After  proper  treatment  which  reduces  the  thread  to 
a  cellulose  or  pulplike  form,  it  is  bent  around  blocks  of  carbon  and  is 
packed  in  a  crucible  filled  with  powdered  carbon.  The  material  is  then 
converted  into  black  carbon  hairpins  by  baking  for  many  hours.  The 


FIG.  190.  —  Geissler 
Pump. 


FIG.  191.  —  Sprengel 
Pump. 


ARC  AND   INCANDESCENT   LIGHTING  285 

hairpin  form  comes  from  the  shape  of  the  blocks  around  which  the 
material  was  wrapped. 

To  bring  the  filaments  to  the  proper  resistance  and  at  the  same  time 
put  them  into  condition  to  stand  the  strain  of  the  high  temperature  of 
"  burning,"  they  are  commonly  "  treated  "  by  a  process  which  deposits 
very  hard  gray  carbon  upon  their  surfaces.  This  treatment  is  usually 
termed  "  flashing,"  and  consists  in  immersing  the  filaments  in  naphtha 
gas  or  petroleum  and  passing  a  current  through  them.  The  current 
heats  the  thin  parts  of  the  filaments  to  a  white  heat.  This  heat  in  turn 
decomposes  the  naphtha  or  petroleum,  and  carbon  is  deposited  on  the 
filament. 

The  filaments  are  then  each  mounted  upon  two  short  pieces  of 
platinum  wire  which  are  sealed  into  a  bit  of  glass.  The  connection 
between  the  carbon  and  the  platinum  is  usually  made  satisfactory  from 
an  electrical  point  of  view  by  means  of  a  cement.  The  filament  thus 
mounted  is  sealed  into  the  bulb  by  a  glass-blower  in  the  way  described 
in  the  next  paragraph. 

The  bulbs  are  usually  purchased  ready  made  from  a  glass  factory. 
One  of  these  bulbs  is  selected  and  a  piece  of  glass  tube  is  connected  to 
the  top  of  the  bulb.  This  serves  as  a  handle  for  the  workmen  and  also 
for  connecting  the  lamp  to  the  pump.  The  carbon  filament  is  then  in- 
serted into  the  neck  of  the  bulb,  and  the  glass  at  the  base  of  the  carbon 
is  so  carefully  welded  into  the  glass  .base  of  the  bulb  that  the  union 
becomes  absolutely  perfect.  After  exhausting,1  the  glass  tube  at  the  top 
of  the  bulb  is  sealed  off  and  the  lamp  is  complete.  The  wires  passing 
through  the  glass  are  of  platinum,  because  that  is  the  only  material  now 
known  that  will  maintain  a  tight  joint.  The  little  point  usually  found  at 
the  top  of  an  incandescent  lamp  is  the  stub  which  is  left  when  the  glass 
tube  is  sealed  off  after  the  lamp  has  been  exhausted. 

258.  Lamp  Bases  and  Sockets.  —  For  convenience  in  use,  incandes- 
cent lamps  are  mounted  on  bases  to  which  they  are  fastened  with 
plaster.  These  bases  contain  two  contacts  which  correspond  to  two 
contacts  in  a  Socket  which  may  be  connected  to  an  electric  circuit. 
In  Figure  192,  b  is  the  lamp  bulb,  c  is  the  carbon  filament,  ww  are 
the  platinum  leading-in  wires,  /  is  the  cement  connecting  the  carbon 

1  Article  256. 


286 


ELECTRICITY   AND   MAGNETISM 


FIG.  192.  —  Diagram 
of  Incandescent 
Lamp. 


and  platinum,  f  is  the  brass  base  which  is  attached  to  the  lamp  by  the 

plaster  /,  d  and  r  are  the  two  contacts  by  which  the  carbon  is  brought 
into  connection  with  the  electric  circuit  when  the 
lamp  is  inserted  in  a  socket,  and  /  is  the  point  where 
the  lamp  was  "sealed  off"  the  pump.  The  bases 
used  on  lamps  have  various  external  forms  depend- 
ing upon  the  manufacturer,  and  the  one  shown  in  the 
figure  is  very  commonly  used. 

In  order  that  incandescent  lamps  may  be  as  con- 
veniently turned  on  and  off  as  gaslights,  the  sockets 
often  contain  switches  as  shown  in  Figure  193,  which 
is  a  skeleton  view  of  a  socket.  Where  lamps  are 
arranged  to  be  controlled  by  wall  switches,  plain  or 
Keyless  sockets  are  generally  used. 

259.  Parallel  and  Series  Connections  for  Electric 
Lamps  or  Motors.  —  Incandescent  electric  lamps  and 

electric  motors  are  sometimes  operated  upon  series  circuits,  but  they 

are  much  more  satisfactory  when  connected  in  parallel,1  as  is  usually  done. 
The  difference  between  the  connection  of 

lamps  in  parallel  and  lamps  in  series  may 

be  illustrated  by  comparing  the  methods  of 

utilizing  water  power.     Suppose  a  series  of 

dams  is  placed   in  a  stream  and  a  mill  is 

placed    at    each    dam.      The    water   which 

passes  through  the  water-wheels  of  the  first 

mill   flows    down    to  the    second    mill    and 

passes  through  its  wheels,  and  thus  contin- 
ues to  flow  through  the  wheels  of  one  mill 

after  another.     The  wheels  of  each  mill  are, 

therefore,  turned   by   the    same   water  that 

turns   the   wheels  of  every  other   mill.     In 

order  that  this  may  be  the  condition,  each 

mill  must  be  located  on  a  lower  level  than 

the  one  up  stream  from  it.     Then  the  total  fall  of  the  stream  is  so 

divided  that  each  mill  gets  advantage  of  a  proper  proportion. 

1  Article  103. 


•FlG.  193.  —  Skeleton  Diagram 
of  Lamp  Socket. 


ARC  AND   INCANDESCENT   LIGHTING  287 

In  series  arc  lighting  the  same  current  flows  through  all  the  lamps  one 
after  the  other,  and  the  total  pressure  at  the  dynamo  is  divided  amongst 
the  lamps.  If  an  arc  dynamo  is  capable  of  producing  1000  volts,  it  will 
operate  twenty  open  arc  lamps  in  series,  since  it  takes  about  fifty  volts  to 
send  the  current  through  each  arc.  If  a  portion  of  the  lamps  are  cut 
out  of  circuit,  the  pressure  at  the  dynamo  must  be  reduced  or  the 
current  will  increase  above  its  proper  value. 

If,  instead  of  dividing  up  the  total  fall  of  the  stream  so  that  each  mill 
gets  the  benefit  of  a  part,  a  large  dam  is  built  on  the  stream  and  the 
mills  are  located  so  that  they  all  take  water  from  the  same  canal  and 
discharge  water  into  the  same  tailrace,  all  of  the  mills  get  the  benefit  of 
the  entire  fall,  but  the  water  of  the  stream  is  divided  between  them  in 
proportion  to  their  needs,  and  their  wheels  are  in  parallel.  The  amount 
of  water  flowing  through  the  wheels  of  each  mill  in  this  case  is  directly 
proportional  to  the  work  being  done  in  that  mill.  If  one  mill  is  shut 
down,  the  gate  through  which  water  is  admitted  to  the  wheel  is  closed, 
and  no  water  flows  through.  The  water  used  by  each  mill  is  entirely 
independent  of  the  amount  used  by  the  others. 

In  the  same  way,  when  electric  lamps  are  connected  in  parallel  the 
current  flowing  through  each  lamp  is  entirely  independent  of  that  flow- 
ing through  the  others,  and  simply  depends  upon  the  resistance  of  the 
lamp  and  the  pressure  at  its  terminals.  When  it  is  desired  to  cut  out 
of  circuit  a  lamp  which  is  connected  in  parallel  with  others,  its  con- 
nection with  the  circuit  is  broken  by  a  switch  (Fig.  193)  so  that  no 
current  can  flow  through  it.  This  is  equivalent  to  closing  the  gate 
through  which  water  enters  a  mill,  as  already  explained. 

When  it  is  desired  to  shut  down  one  of  a  number  of  mills  in  series,  it 
evidently  will  not  do  to  simply  close  the  gates  which  admit  water  to  the 
wheels,  as  that  would  prevent  the  water  from  flowing  to  the  other  mills, 
but  it  is  necessary  to  arrange  a  short  path  for  the  water  to  flow  around 
the  mill  which  is  shut  down.  In  the  same  way,  when  it  is  desired  to 
turn  off  an  electric  lamp  which  is  operated  in  a  series  circuit,  the  lamp 
must  be  short-circuited  as  at  A  in  Figure  194.  Some  special  switches 
used  on  arc-lighting  circuits  short-circuit  the  lamp  which  is  to  be  turned 
off",  so  that  the  main  line  is  properly  completed,  and  then  also  discon- 
nect the  lamp  terminals  from  the  line. 


288 


ELECTRICITY   AND   MAGNETISM 


FlG.  194.  —  Illustration  of 
Series  Circuit  with  One 
Lamp  cut  out. 


260.  Effect  of  a  Change  of  Pressure  on  an  Incandescent  Lamp.  — 
Since  the  current  which  flows  through  incandescent  lamps  connected  in 

parallel  depends  upon  the  pressure  at  the  lamp 
terminals,  and  the  light  given  by  each  filament 
depends  upon  the  current  flowing  through  it, 
the  pressure  at  the  terminals  of  the  lamps  must 
be  kept  perfectly  constant,  or  they  will  not  give 
a  steady  light.  If  the  electrical  pressure  at  the 
terminals  of  an  incandescent  lamp  is  changed, 
the  light  given  off  by  the  filament  changes  at  a 
much  faster  rate. 

If  a  lamp,  for  instance,  which  is  intended  for 
a  pressure  of  no  volts  and  to  give  16  candle 
power,  is  connected  to  a  105  volt  circuit,  the 

light  which  it  gives  is  no  more  than  about  12  candle  power  and  is  of  a 
poor  red  color.  If  the  same  lamp  is  connected  to  a  115  volt  circuit, 
the  light  which  it  gives  becomes  about  20  candle  power  and  is  of  a 
brilliant  whitish  color.  The  great  candle  power  and  whiteness  of  the 
light  in  the  latter  case  shows  that  the  filament  is  so  excessively  hot 
that  even  refractory  carbon  cannot  last  long  under  the  strain,  and  the 
filament  will  soon  give  out. 

The  length  of  time  during  which  the  filament  of  an  incandescent  lamp 
will  last  —  that  is,  the  Life  of  the  lamp  —  decreases  very  rapidly  as  the 
temperature  at  which  the  filament  burns  is  increased  above  its  proper 
value.  On  the  other  hand,  the  power  required  to  produce  light 
increases  as  the  working  temperature  of  the  filament  decreases.  It 
should,  therefore,  always  be  the  aim  to  operate  incandescent  lamps  at 
the  exact  pressure  for  which  they  were  designed. 

261.  Distributing  Wires.  —  We  have  already  seen  that  there  is  always 
a  loss  of  pressure  when  an  electric  current  flows  through  a  wire  —  this 
loss  being  equal   to   the  product  of  the  amperes  of  current   and  the 
resistance  of  the  wire.1     Consequently,  when  a  number  of  incandescent 
lamps  in  parallel  are  connected  to  a  circuit  at  some  distance  from  the 
dynamo  which  supplies  the  current,  the  wires  of  the  circuit  must  be 
quite  heavy  in  order  that  the  loss  in  pressure  shall  not  be  too  great. 

1  Articles  92  and  106. 


ARC  AND   INCANDESCENT   LIGHTING  289 

When. incandescent  lamps  and  motors  are  connected  to  wires  which 
lead  from  a  central  generating  station,  it  is  common  to  allow  a  loss  of 
pressure  or  "  drop  "  amounting  to  as  much  as  ten  to  twenty  per  cent  of 
the  dynamo  pressure  when  all  the  lamps  are  turned  on.  The  circuits 
must  be  so  arranged  that  all  the  lamps  shall  be  fed  with  current  at  as 
nearly  the  same  pressure  as  possible,  and  if  the  "  drop  "  is  allowed  to  be 
greater  than  twenty  per  cent,  this  becomes  a  difficult  matter. 

When  the  "  drop  "  in  the  wires  is  too  great,  it  is  also  difficult  to  regulate 
the  dynamos  so  that  the  pressure  at  the  lamps  shall  not  vary  when  lamps 
are  turned  on  or  off.  It  is  easy  to  see  that  every  lamp  which  is  turned 
on  or  off  changes  the  current  flowing  through  the  wires  of  the  circuit, 
and  therefore  changes  the  pressure  lost  in  the  wires  between  the 
dynamos  and  the  lamps.  In  plants  which  are  confined  to  a  single  build- 
ing the  "  drop,"  when  all  the  load  (lamps  and  motors)  is  turned  on,  is  usu- 
ally made  to  be  from  two  to  eight  per  cent  of  the  pressure  at  the  dynamo. 

The  transmission  of  power  to  the  lamps  will  be  dealt  with  at  greater 
length  in  a  following  chapter. 

QUESTIONS 

1.  Who  first  produced  the  electric  arc?      When? 

2.  Why  does  an  arc  continue  to  burn  after  the  carbons  are  separated  a  little  ? 

3.  What  is  the  crater  of  an  arc?      What  makes  it? 

4.  Why  is  the  positive  carbon  put  above  the  negative  when  the  electric  arc  is 
used  for  lighting? 

5.  Does  the  positive  carbon  burn  more  or  less  rapidly  than  the  negative? 

6.  What  duties  must  an  arc-lamp  mechanism  perform? 

7.  Explain  how  the  two  coils  of  an  arc-lamp  mechanism   keep  the  arc  of  the 
proper  length. 

8.  What  is  a  differential  magnet? 

9.  Describe  the  enclosed  arc  lamp. 

10.  How  does  an  enclosed  arc  lamp  differ  from  an  open  one? 

11.  What  are  the  ordinary  pressures,  currents,  and  candle  powers  of  open  arc 
lamps? 

12.  Wrhat  is  a  candle  power? 

13.  What  is  the  character  of  the  distribution  of  the  light  around  a  direct-current 
arc  lamp  ? 

14.  What  are  double  arc  lamps? 

15.  How  are  arc-light  carbons  made?     What  sizes  are  commonly  used  ? 


2QO  ELECTRICITY   AND    MAGNETISM 

1 6.  How  are  arc  dynamos  regulated  to  give  constant  currents? 

17.  Describe  an  arc-lighting  switchboard. 

18.  What  happens  if  the  voltage  of  an  arc  lamp  is  too  high?     If  too  low? 

19.  Give  a  short  history  of  the  development  of  the  incandescent  lamp. 

20.  Why  is  carbon  used  for  incandescent  lamp  filaments? 

21.  Why  is  the  bulb  of  an  incandescent  lamp  exhausted? 

22.  How  are  lamp  bulbs  exhausted? 

23.  How  are  carbon  filaments  made? 

24.  What  is  flashing? 

25.  Why  is  platinum  used  for  the  conducting  wires  which  lead  in  through  the 
glass  of  incandescent  lamp  bulbs? 

26.  Describe  an  incandescent  lamp  base  and  a  socket. 

27.  Why  must  the  pressure  of  an  arc  dynamo  vary  with  the  number  of  series 
lamps  in  circuit? 

28.  Give  a  water  analogy  to  a  series  system  of  lighting. 

29.  Give  a  water  analogy  to  a  parallel  system  of  lighting. 

30.  How  can  a  series  lamp  be  cut  out  of  circuit?     How  can  a  parallel  lamp? 

31.  Why  is  it  absolutely  necessary  to  work  incandescent  lamps  at  their  proper 
pressure  to  give  satisfaction? 

32.  Why  must  the  "  drop  "  in  the  conductors  of  parallel  lighting  systems  be  kept 
small? 

33.  How  can  the  "  drop  "  in  the  wires  be  kept  small? 

34.  W7hat  is  the  maximum  "drop"  that  should  be  allowed  in  a  parallel  system  of 
lighting? 


CHAPTER   XVIII 

POWER  STATIONS,  THE    ELECTRIC    RAILWAY,   AND   OTHER  APPLI- 
CATIONS  OF   MOTORS 

262.  Development  of  the  Central  Station.— When,  in  1884,  Mr.  J.  E. 
Gordon  wrote  bis  book  called  "A  Practical  Treatise  on  Electric  Light- 
ing," he  filled  the  rather  large  volume  with  descriptions  of  dynamos  and 
electric  lamps  made  in  forms  which  are  now  nearly  all  discarded,  but  at 
that  time  there  was  little  else  to  write  about  in  the  field  of  electric  light- 
ing. There  were  at  that  time  no  great  electric  lighting  plants  such  as 
we  have  to-day,  nor  were  there  any  even  to  be  compared  with  those  in 
existence  only  five  years  later  than  the  date  of  the  book.  With  the  same 
courage  and  optimism  which  led  him  to  say  in  1881,  "  the  day  will  come 
when  gaslight  will  be  as  obsolete  as  wooden  torches,  and  when  in  every 
house  the  incandescent  lamp  will  have  replaced  the  gas-jet,"  Mr.  Gordon 
left  space  in  his  book  for  a  chapter  called  "  Central  Station  Lighting." 
Under  the  heading  was  only  the  single  paragraph,  "  I  had  intended  to 
write  a  long  chapter  with  the  above  heading,  but  for  various  reasons 
I  am  not  yet  prepared  to  do  so.  I  have,  however,  left  in  the  heading 
for  the  convenience  of  inserting  such  a  chapter  in  a  future  edition  of 
this  book,  should  one  ever  be  required." 

At  the  present  time,  less  than  two  decades  later  than  the  time  when  Mr. 
Gordon  wrote,  we  have  numbers  of  books  upon  the  subjects  of  electric 
lighting  and  electric  plants,  and  the  progress  of  this  period  has  been  so 
enormous  that  many  of  the  descriptions  in  Gordon's  book  seem  to  belong 
to  another  age.  We  may  say,  indeed,  that  they  do  belong  to  another 
age,  for  a  decade  constitutes  an  epoch  in  the  history  of  the  modern 
development  of  electrical  apparatus. 

It  is  instructive  and  interesting  to  see  the  way  in  which  electric  plants 
have  developed  since  1884.  This  is  best  shown  by  figures  representing 

291 


ELECTRICITY   AND   MAGNETISM 

plants  which  were  built  at  different  periods.  Figure  195  shows  one  of 
the  earliest  central- station  electric-light  plants  of  the  world,  the  first 
Edison  central  station  for  the  public  supply  of  electric  current,  which  was 
located  at  Appleton,  Wis.,  in  1881.  At  the  left  hand  of  the  figure  is 
shown  the  exterior  of  a  small  frame  shanty  in  which  this  plant  was 
located,  while  at  the  right  hand  of  the  figure  the  shanty  is  shown  with 
one  side  removed,  so  that  the  plant  with  its  dynamo,  pulleys,  and  belts 
is  exposed  to  view. 

This  plant  was  operated  by 'water  power,  and  the  gears  on  the  water- 
wheel  shaft  used  to  drive  the  counter  shafts  to  which  the  dynamo  was 
belted  are  shown  in  the  centre  of  the  figure.  The  plant  was  put  in 
operation  before  the  day  of  the  three-wire  system,1  and  it  therefore  had 


FIG.  195.  — Early  Edison  Electric  Light  Central  Station. 

only  one  dynamo.     Behind  the  dynamo  in  the  picture  the  regulating  and 
indicating  apparatus  are  vaguely  seen. 

A  peculiar  and  interesting  point  in  the  picture  is  the  dynamo,  which, 
it  will  be  noticed,  looks  quite  different  from  those  illustrated  in  preced- 
ing chapters.  This  dynamo  has  a  spindling,  lean  appearance  which  forms 
a  decided  contrast  to  the  chunky,  substantial  form  of  the  modern  dyna- 
mos. The  field  magnets  of  the  dynamo,  which  is  bipolar,  are  divided 
into  several  legs,  as  though  there  were  several  horseshoe  electromagnets 
attached  to  the  poles.  At  the  time  these  machines  were  built  this  was 
supposed  to  be  the  best  way  of  constructing  dynamos,  but  the  modern 
construction  with  a  single  short  horseshoe  has  been  proved  to  be  the 
best  form  for  bipolar  dynamos  with  salient  poles. 

1  Article  334. 


ELECTRIC   POWER   STATIONS 


293 


One  of  these  so-called  "  spindle-shank  "  dynamos  which  was  used  by 
Mr.  Edison  in  his  first  public  exhibition  of  incandescent  electric  lights 
at  Menlo'Park  in  1880  is  now  in  the  dynamo  collection  of  the  University 
of  Wisconsin,  where  it  makes  a  striking  contrast  to  the  appearance  of 
the  substantial  later  dynamos  of  equal  capacity  which  stand  by  its  side. 
Notwithstanding  its  peculiar  appearance,  the  old  dynamo  is  still  good  for 
any  reasonable  service,  and,  indeed,  it  had  been  doing  almost  daily 
work  from  1880  up  to  the  time  of  the  World's  Fair  in  Chicago,  where  it 
was  exhibited,  and  from  whence  it  was  forwarded  to  its  present  place. 


FIG.  196.  —  Old  Style  and  Ne\v  Style  Bipolar  Edison  Dynamos. 

One  of  the  old  spindle-shanks,  with  a  later  dynamo  of  the  same  capacity 
beside  it,  is  shown  in  Figure  196. 

The  plant  which  is  now  located  at  Appleton,  Wis.,  is  as  great  a  con- 
trast to  the  original  one  as  the  old  dynamo  is  to  modern  machines.  It 
now  contains  several  fine  dynamos  with  excellent  regulating  devices, 
housed  in  a  substantial  building,  which  are  used  to  furnish  current  to 
incandescent  and  arc  lights,  stationary  electric  motors,  and  to  electric 
cars. 

263.  The  Pearl  Street  Station.  —  The  great  landmark  in  electric  cen- 
tral stations,  the  Pearl  Street  station  of  New  York  City,  was  operated 
continuously  from  the  fall  of  1882  until  1894,  when  it  was  destroyed  by 


294  ELECTRICITY   AND    MAGNETISM 

fire.  It  has  now  been  replaced  by  a  magnificent  station,  to  which  refer- 
ence will  be  made  later.  Figure  197  shows  one  of  the  then  great 
"Jumbo"  dynamos  which  were  used  in  this  station,  each  directly  coupled 
to  its  own  engine.  Each  one  of  these  dynamos  had  a  capacity  of  1500 
sixteen-candle-power  incandescent  lamps,  and  occupied  not  less  than  1 75 
square  feet  of  floor  space.  It  is  interesting  to  compare  the  "Jumbo" 
machine  with  one  of  the  latest  triumphs  of  electrical  engineering,  the  great 


FIG.  197.  —  Old  "  Jumbo  "  Dynamo  with  its  Engine. 

"steam  dynamo"  shown  in  Figure  198,  which  has  a  capacity  of  20,000 
sixteen-candle-power  incandescent  lamps,  and  occupies  but  little  more 
floor  space  than  the  "Jumbo."  The  "Jumbo"  dynamos  were  wonder- 
ful machines  in  their  day,  and  a  few  were  running  in  European  electric- 
light  stations  until  quite  lately  ;  but  most  of  them  were  superseded,  soon 
after  their  introduction,  by  faster  running  central-station  dynamos,  driven 
by  leather  belts  instead  of  being  directly  coupled  to  engines. 


ELECTRIC   POWER   STATIONS 


295 


264.  The  Vertical  Station.  —  This  move  to  dynamos  driven  by  means 
of  belts  caused  a  change  in  the  arrangements  of  city  central  stations,  so 
that  several  great  plants  built  in  New  York,  Chicago,  Philadelphia,  and 
Boston  were  constructed  after  the  general  plan  shown  in  Figure  199. 
This  figure  shows  a  cross  section  of  one  of  the  central  stations  of  the 
Edison  Electric  Illuminating  Company  of  New  York  City.  Here  high- 
speed steam  engines  are  located  in  the  basement  of  the  building,  so  that 


FIG.  198.  —  Modern  Dynamo  with  its  Engine. 

they  may  be  on  a  solid  foundation,  and  driving  belts  run  from  their  fly- 
wheels to  dynamos  located  upon  the  floor  above.  The  two  floors  above 
the  dynamos  are  occupied  by  boilers  which  furnish  steam  to  the  engines 
located  in  the  basement,  and  by  arrangements  for  handling  the  ashes 
which  come  from  the  boiler  furnaces.  Above  the  boilers  is  a  floor  wholly 
given  over  to  bins  for  holding  coal  for  the  boilers,  which  is  hoisted  from 
the  street  by  an  elevator.  The  top  floor  is  given  to  repair  shops,  store- 
rooms, etc. 


296 


ELECTRICITY   AND    MAGNETISM 


FIG.  199.  —  Vertical  Cross  Section  of  Early  Large  Station. 


ELECTRIC   POWER   STATIC^. 


297 


This  central  station  fairly  represents  the  type  which  was  used  for  a 
number  of  years  in  great  cities,  where,  on  account  of  the  expense  of 
land,  it  is  desirable  to  occupy  as  little  ground  space  as  possible.  In  the 
great  stations  which  have  been  built  in  Chicago,  Boston,  and  New  York 
within  the  last  half  decade,  the  arrangement  is  still  more  economical. 
This  will  be  referred  to  later. 

265.  Plants  in  Small  Cities.  —  In  the  smaller  cities,  or  places 
where  land  is  not  so  valuable,  it  has  been  usual  to  place  the  boilers 


FIG.  200.  —  Plan  of  Central  Station. 

on  the  ground  floor  with  the  engines,  and  the  dynamos  are  then  placed 
either  upon  the  same  floor  or  on  the  floor  above. 

One  arrangement  of  a  central  station,  with  the  boilers,  engines,  and 
dynamos  all  on  the  same  floor,  is  well  shown  in  Figure  200.  Four 
engines  are  shown  in  this,  with  a  dynamo  driven  by  a  belt  from  each 
fly-wheel. 

A  station  with  boilers  and  engines  on  one  floor  and  the  dynamos  on 
the  floor  above  is  very  well  shown  in  Figure  201,  which  is  a  cross  section 
of  a  large  plant.  The  dotted  lines  in  the  figure  show  where  an  addi- 
tional engine,  with  its  counter  shaft  and  set  of  dynamos,  may  be  placed. 


298 


ELECTRICITY   AND   MAGNETISM 


These  figures  are  taken  from  actual  plants  which  are  in  successful 
operation,  and  their  counterparts  may  be  seen  in  a  great  many  cities 
and  towns  in  this  country. 


FIG.  201.  —  Vertical  Cross  Section  of  Central  Station. 

266.  The  Niagara  Plant.  —  After  several  years,  during  which  small 
dynamos  were  used  in  electric  plants,  belted  to  counter  shafts  or  directly 
to  the  fly-wheels  of  engines,  the  manufacturers  of  dynamos  began  again 
to  make  dynamos,  which,  like  the  "Jumbo  "  machines,  could  be  directly 
connected  to  engines,  and  such  machines  are  now  generally  used  in 
central  stations.  An  illustration  of  a  dynamo  with  its  engine  is  shown 
in  Figure  198.  The  greatest  machines  of  the  kind  built  until  very 
recently  are  the  great  dynamos  which  are  erected  in  the  power  house 
of  the  Niagara  Falls  Power  Company,  at  Niagara  Falls. 

The  works  of  this  company  constitute  the  greatest  industrial  power 
plant  ever  constructed.  The  location  of  the  plant  is  shown  in  Fig- 
ure 202.  Taking  water  from  the  Niagara  River  above  the  falls,  a  canal 
built  for  the  power  company  by  the  Cataract  Construction  Company 
conducts  the  water  about  1500  feet,  to  where  the  water-wheels  are 


NIAGARA   FALLS   POWER   COMPANY 


299 


located.  These  wheels  are  placed  at  the  bottom  of  two  enormous  wheel 
pits  1 79  feet  deep,  2 1  feet  wide,  and  of  sufficient  length  to  permit  the 
location  of  many  very  powerful  turbine  water-wheels.  The  water  is  con- 
veyed from  the  canal  on  the  surface  of  the  ground  down  to  the  wheels 
at  the  bottom  of  the  pit,  through  great  steel  tubes  or  "  penstocks," 
which  are  y|  feet  in  diameter.  After  the  water  has  passed  through  the 


FIG.  202.  —  Map  of  Niagara  Falls,  showing  Plant  of  Niagara  Falls  Power  Company. 

water-wheels,  delivering  up  to  them  its  power,  it  is  carried  away  through 
a  tunnel  a  mile  and  a  quarter  long,  to  be  discharged  into  the  river  below 
the  falls. 

The  canals  and  tunnels  of  the  Niagara  Falls  Power  Company  have 
been  constructed  on  such  a  scale  that  the  amount  of  water  which  will 
pass  through  them  is  capable  of  delivering  125,000  horse  power  to  the 


300  ELECTRICITY   AND    MAGNETISM 

water-wheels,  and  the  charter  of  the  company  permits  it  to  take  more 
water  to  about  an  equal  amount.  The  amount  of  power  represented  by 
this  is  as  much  as  one-tenth  of  the  power  which  can  be  developed  by  all 
the  water-wheels  in  the  United  States,  and  is  greater  than  the  water 
power  of  the  following  great  power  and  manufacturing  centres  all  added 
together :  Lawrence,  Lowell,  and  Holyoke,  Mass. ;  Manchester,  N.H. ; 
Lewiston,  Me.  ;  Bellows  Falls,  Vt. ;  Rochester,  Cohoes,  Oswego,  and 
Lockport,  N.Y. ;  Paterson,  NJ. ;  Augusta,  Ga.  ;  and  Minneapolis,  Minn. 

Even  this  enormous  amount  of  power  which  the  Niagara  Falls  Power 
Company  proposes  to  supply  to  its  customers  is  very  small  compared 
with  the  power  which  is  contained  by  all  the  water  in  the  falls.  If  all 
the  power  represented  by  the  water  as  it  flows  through  the  upper  rapids 
of  the  Niagara  River,  over  the  falls,  and  through  the  lower  rapids,  were 
utilized,  it  is  estimated  that  it  would  make  about  7,000,000  horse  power, 
or  perhaps  three  times  as  much  as  the  power  of  all  the  water-wheels  in 
this  country,  and  considerably  more  than  the  combined  power  of  all  the 
steam  engines  and  water-wheels  which  are  used  in  the  country. 

The  Niagara  Falls  Power  Company  is  unable  to  take  advantage  of 
the  total  height  down  which  the  water  flows  ;  but  if  the  power  of  all  the 
water  in  the  falls  were  as  fully  utilized  as  the  power  company  utilizes 
the  power  of  the  water  which  passes  through  its  wheels,  it  is  estimated 
that  it  would  still  yield  4,000,000  horse  power,  or  much  more  than  half 
of  all  the  power  now  used  in  this  country. 

It  is  therefore  to  be  seen  that  the  great  plans  of  the  Niagara  Falls 
Power  Company,  when  fully  carried  out,  are  estimated  to  divert  only 
about  one-sixteenth  of  all  the  water  from  the  falls,  and  plenty  will  remain 
for  the  purposes  of  other  power  companies,  as  well  as  to  maintain  the 
grandeur  and  beauty  of  the  falls  unimpaired. 

267.  Construction  of  the  Niagara  Plant.  —  In  Figures  203  and  204 
are  shown  two  sketch  views  of  the  wheel  pit  and  power  house  of  the 
Niagara  Falls  Company.  The  first  figure  shows  a  vertical  section  taken 
crosswise  through  the  wheel  pit  and  house,  and  the  second  shows  a 
vertical  section  taken  lengthwise  through  a  part  of  the  pit  and  house, 
and  shows  the  positions  of  two  of  the  water-wheels  and  dynamos.  In 
the  lower  left-hand  corner  of  the  latter  figure  is  seen  the  tailrace  tunnel 
by  which  the  water  is  discharged  into  the  river.  In  the  figures,  W\V 


NIAGARA   FALLS   POWER   COMPANY 


301 


are  the  water-wheels,  each  of  which  is  composed  of  two  twin  wheels 
having  together  the  enormous  capacity  of  5000  horse  power,  and  PPare 
the  penstocks.  SS  are  great  hollow  steel  shafts  no 
less  than  thirty-eight  inches  in  diameter.  Each 
shaft  conveys  the  5000  horse  power  developed  by 
the  water-wheel,  to  which  it  is  attached,  to  a  great 
dynamo  fastened  to  its  upper  end.  At  C,  in  Fig- 
ure 203,  the  canal  which  brings  water  to  the  pen- 
stocks is  shown,  and  at  T  is  shown  the  electric 
travelling  crane,  capable  of  lifting  fifty  tons,  which 
is  placed  in  the  power  house  to  be  used  for  plac- 
ing the  machinery  in  position,  and  for  taking  the 
machinery  to  pieces  if  this  becomes  necessary  at 
any  time  for  the  purpose  of  repairs. 

The  5000  horse-power  water-wheels,  which  are 
over  five  feet  in  diameter  and  revolve  at  a  speed 
of  250  revolutions  per  miriute,  are  marvels  of  en-  FIG.  203. Vertical  Sec- 

gineering  and  constructive  skill,  but  we  cannot  stop     tion  across  Wheel  Pit 

.  ,          .,  i        .,         of  Niagara  Falls  Powt^r 

to     consider    their    details     company 

or  the  remarkable  bearings 
upon  which  are  supported  the  enormous  weights 
of  the  revolving  parts  of  dynamo  and  shaft  that 
are  connected  to  each  wheel,  and  that  amount 
to  a  total  of  some  forty  tons.  Suffice  it  to  say 
that  the  upward  pressure  of  the  water  itself  is 
so  nicely  balanced  as  to  almost  exactly  over- 
come this  weight  when  the  wheels  are  running. 

Each  dynamo,  as  a  whole,  weighs  eighty  tons 
and  is  of  the  most  massive  character.  These 
dynamos  generate  a  two-phase  alternating  cur- 
rent,1 at  2000  volts  pressure,  with  the  quite  low 
frequency  of  twenty-five  periods  per  second  ; 
and  the  currents  are  used  for  operating  either 
motors  or  lights.  As  the  plant  is  primarily  de- 
signed for  the  transmission  of  power  to  factories 


FIG.  204.  —  Vertical  Sec- 
tion along  Wheel  Pit  of 
Niagara  Falls  Power 
Company. 


1  Article  246. 


302 


ELECTRICITY   AND    MAGNETISM 


and  mills,  it  is  expected  that  the  greater  part  of  the  current  will  be 
used  in  operating  motors.  Thus  far  only  ten  generating  units,  of  5000 
horse  power  each,  have  been  installed  in  the  electric  power  house, 
although  much  additional  water  power  is  now  being  furnished  directly 
to  paper  mills. 

A  part  of  the  interior  of  the  electric  power  house  is  shown  in 
Figure  205.  Three  of  the  great  dynamos  and  the  end  of  one  of  the 
switchboards  are  clearly  indicated.  The  frame  of  the  dynamo  field 


FlG.  205.  —  Part  of  Interior  of  Station  of  Niagara  Falls  Power  Company. 

magnets,  which  compose  the  revolving  part,  is  a  ring  of  forged  nickel 
steel,  made  by  the  Bethlehem  Steel  Company,  the  great  manufacturers 
of  nickel-steel  armor  plate  for  the  government  men-of-war.  The  pole- 
pieces,  twelve  in  number,  are  of  soft  steel,  bolted  to  the  inside  of  the 
ring,  and  wound  with  rectangular  wire.  The  armature  is  built  up  of 
disks  of  soft  steel,  and  the  windings,  of  rectangular  section,  are  placed  in 
slots.  The  armature  is  stationary  and  inside  of  the  circumference  of  the 
revolving  field. 


NIAGARA   FALLS   POWER  COMPANY  303 

The  first  customer  to  which  electric  power  was  delivered  when  the 
great  dynamos  were  put  into  service  was  the  Pittsburg  Reduction  Com- 
pany, whose  works  for  the  production  of  aluminum  by  electrometallurgy 
were  moved  from  Pittsburg,  Pa.,  to  Niagara,  in  order  to  take  advantage  of 
cheap  electric  power.  Many  other  manufactories  quickly  followed,  and 
quite  a  colony  of  large  mills  and  factories  are  gathered  about  the 
Niagara  electric  power  house.  The  Calcium  Carbide  Works  are  said  to 
alone  take  13,000  horse  power  from  this  plant.  The  current  is  dis- 
tributed to  these  adjacent  mills  at  2000  volts  pressure. 

Power  is  also  transmitted  to  the  city  of  Buffalo,  twenty  miles  away,  at 
a  pressure  of  22,000  volts.  To  obtain  this  pressure,  cables  are  laid  from 
the  station  switchboard  to  a  transformer1  room,  where  the  pressure  is 
raised  and  the  system  changed  by  proper  transformer  connections  from 
two  to  three  phases.  The  current  is  then  carried  out  upon  the  pole  line 
to  the  city  of  Buffalo,  where  it  is  again  reduced  in  pressure  by  huge 
transformers,  placed  in  transformer  houses,  for  distribution  through  the 
city.  At  the  present  time  the  street  railways,  electric  lights,  and  many 
of  the  factories  and  grain  elevators  of  Buffalo  are  operated  by  electric 
power  from  Niagara  Falls.  Where  direct  currents  are  required,  as  for 
the  operation  of  street  railways,  rotary  converters 2  are  used.  The  pole 
line  which  supports  the  transmission  feeders  is  partly  in  duplicate. 

Though  the  plant  at  Niagara  has  now  a  capacity  of  only  50,000  horse 
power,  another  power  house  is  being  erected  which  will  double  the  out- 
put. After  a  time  it  is  possible  that  power  may  be  furnished  from  the 
Niagara  plant,  as  has  been  proposed,  for  the  purpose  of  propelling  canal 
boats  on  the  Erie  Canal,  and  for  manufacturing  purposes  in  cities  as  far 
from  Niagara  as  Rochester,  Syracuse,  and  Albany.  For  the  transmission 
of  power  over  these  long  distances,  the  pressure  at  which  the  current  is 
supplied  to  the  lines  may  be  raised  by  means  of  transformers  to  50,000 
volts  or  even  higher;  but  before  entering  the  consumers'  premises  it  must 
be  reduced  to  a  safe  value,  again  by  means  of  transformers.  Many  of  the 
proposals  that  have  been  made  in  the  newspapers  in  regard  to  the  trans- 
mission of  power  from  Niagara  are  manifestly  impractical ;  but  many  of 
its  possibilities  may  yet  be  unappreciated,  and  it  is  impossible  to  fore- 
tell the  developments  that  may  occur. 

1  Article  239.  2  Article  248. 


304  ELECTRICITY   AND    MAGNETISM 

The  Niagara  station  will  serve  to  illustrate  present  practice  in  build- 
ing large  plants ;  but  it  is  of  interest  to  add  that  an  electric  generating 
station  of  70,000  horse  power  is  just  being  completed  on  the  banks  of 
New  York  Harbor,  which  is  intended  to  supply  a  portion  of  the  power 
needs  of  New  York  City.  Steam  engines  drive  the  dynamos  in  this 
plant,  and  the  size  of  the  machines,  the  methods  of  electrical  transmis- 
sion and  distribution,  and  the  purposes  of  the  plant  make  it  even  more 
pretentious  than  that  just  described. 

268.  Switchboards. — Before  leaving  the  question  of  central  sta- 
tions, it  is  well  to  examine  the  common  methods  of  handling  dynamos 
in  a  plant  designed  to  furnish  electricity  for  lights  and  power.  The 
current  from  the  dynamos  is  led  to  the  switchboard  by  conducting 
cables  of  the  proper  size,  which  are  connected  to  the  main  switchboard 
conductors,  called  "  bus  bars,"  through  proper  indicating  instruments 
and  switches. 

In  continuous  current,  low-pressure  stations,  where  shunt-wound  dyna- 
mos are  used,  one  dynamo  terminal  is  usually  connected  directly  to 
the  proper  bus  bar  without  the  intervention  of  a  switch,  while  the 
other  dynamo  terminal  is  connected  to  its  bus  bar  through  a  single 
pole  switch.  In  alternating  current  stations,  where  the  dynamos 
furnish  a  pressure  of  1000  volts  or  more,  a  double  pole  switch,  to 
which  both  cables  from  the  dynamo  are  connected,  is  considered 
essential. 

It  is  usual  to  operate  continuous  current  dynamos  in  parallel1  on 
one  set  of  bus  bars,  but  alternators  have  been  almost  always  operated 
on  separate  circuits  in  this  country,  on  account  of  the  difficulty  of  keep- 
ing them  in  step,2  though  the  large  power  stations  of  recent  years  are 
now  all  operating  alternators  in  parallel.  This  makes  quite  a  difference 
in  the  arrangement  of  switchboards  in  the  two  kinds  of  stations.  In 
parallel  running  stations  all  Feeders  3  are  connected  directly  to  the  main 
bus  bars,  but  in  others  the  feeder  switches  are  usually  arranged  so 
that  any  feeder  may  be  individually  connected  to  any  dynamo  as 
desired.  Figure  206  shows  the  switchboard  for  a  direct  current  elec- 
tric lighting  plant,  having  two  dynamos  and  two  lamp  circuits.  The 
wiring,  which  is  behind  the  board,  is  shown  by  means  of  dotted  lines. 
1  Articles  103  and  244.  2  Article  244.  3  Article  328. 


PARALLELING   DYNAMOS 


305 


269.  Cutting  Machines  in  and  out  of  Service.  —  We  will  suppose,  for 
an  example,  a  large  continuous  current  station,  in  which  one  or  two 
engines  with  their  dynamos  have  been  running  all  day  to  supply  the 
demand  for  current  in  the  daytime,  and,  as  evening  approaches,  addi- 


FIG.  206.  —  Outline  of  Front  of  Switchboard  arranged  to  control  Two  Dynamos  and  Two 
Feeders.     The  conductors  used  for  connecting  between  the  switches,  instruments,  and 
other  devices,  are  designed  to  be  on  the  back  of  the  marble  tablet  that  constitutes  the 
board.    These  connecting  conductors  are  indicated  by  dotted  lines. 
X 


306 


ELECTRICITY   AND    MAGNETISM 


VOLTMETER 
> 


AMPERE  METER 


DYNAMO 
GALVANOMETER 


tional  engines  and  dynamos  must  be  put  into  service  to  provide  for  the 
greater  demand  for  current  during  the  hours  of  dusk.  A  short  time 
before  additional  machines  are  likely  to  be  needed,  one  or  more  engines 
with  their  dynamos  are  made  ready  for  running,  and  are  then  started  at 
a  slow  speed  to  warm  them  up. 

After  a  time  one  of  the  sets  is  brought  to  full  speed  and  the  dynamo 
attendant  at  the  switchboard  changes  the  resistance  in  the  field  circuit 

by  means  of   the   dynamo 

BUS  BAR  * 

regulator,  which  is  placed 
on  the  board,  until  the 
lamps  mounted  on  top  of 
the  dynamo  burn  with  ap- 
proximately normal  candle 
power.  The  dynamo  is 
then  ready  to  be  put  into 
circuit  whenever  it  is 
needed.  When  this  time 
comes,  the  switchboard  at- 
tendant connects  the  free 
terminal  of  the  dynamo  to 
a  dynamo  galvanometer 
(Figure  207),  and  moves 
the  dynamo  regulator  un- 
til the  galvanometer  needle 
comes  to  zero.  The  press- 
ure developed  by  the  fresh 
dynamo  is  then  exactly 

equal  to  the  bus-bar  pressure.  The  attendant  now  closes  the  dynamo 
switch,  thus  putting  the  machine  into  circuit,  and  then  moves  the 
regulator  until  the  amperemeter  shows  that  the  dynamo  is  taking  its 
proper  proportion  of  the  load. 

While  this  is  being  done,  another  generating  set  is  brought  to  speed 
and  made  ready  to  go  into  circuit  whenever  it  is  required.  The  opera- 
tion is  repeated  until  all  the  dynamo  capacity  that  is  required  during  the 
period  of  heavy  load  is  in  service.  Some  cities  are  subject  to  sudden 
periods  of  darkness  caused  by  clouds  or  smoke,  and  at  such  times  it 


FIG.  267. —  Illustration   of  the   Use   of  a   Dynamo 

Galvanometer. 


ELECTRIC   RAILWAYS  307 

often  requires  very  prompt  action  on  the  part  of  station  attendants  to 
get  the  dynamos  into  circuit  as  quickly  as  they  are  needed.  Instead  of 
a  special  galvanometer,  which  is  shown  in  the  figure,  a  voltmeter  may  be 
used  for  comparing  the  pressure  of  bus  bars  and  dynamos. 

After  a  period  of  heavy  load  is  over,  the  dynamos  are  withdrawn  from 
the  circuit  and  the  engines  are  shut  down.  When  a  dynamo  is  to  be  with- 
drawn from  the  circuit,  its  regulator  is  moved  until  the  amperemeter 
shows  that  it  carries  very  little  load,  and  the  switch  is  then  opened. 

The  process  of  getting  extra  dynamos  into  service  in  an  alternating 
current  station  is  quite  similar  to  the  preceding,  but  after  the  dynamo 
is  made  ready  to  receive  its  load,  it  may  not  be  put  in  parallel  with 
another  machine,  but  one  or  more  feeders  may  be  transferred  to  it 
from  another  alternator  by  means  of  the  feeder  switches.  If,  however, 
the  alternators  are  designed  to  run  in  parallel,  they  are  not  only  brought 
to  the  same  pressure  as  the  bus  bars,  but  are  also  brought  into  syn- 
chronism and  step.  This  is  done  by  controlling  the  speed  of  the 
engines  a  little,  and  the  proper  conditions  of  pressure,  synchronism,  and 
step  are  indicated  by  an  instrument  which  is  connected  in  the  circuit  in 
the  same  way  as  the  dynamo  galvanometer  which  is  shown  in  Figure  207. 
This  instrument  is  called  a  Synchronizer. 

Synchronizing,  as  it  is  called,  requires  a  good  deal  of  care ;  for  if  the 
machines  are  thrown  together  when  they  are  not  running  at  exactly  the 
same  speed  and  with  their  pulsations  in  unison,  very  bad  flickering  of 
the  lamps  will  result.  After  they  are  running  together  properly  they  tend 
to  keep  in  step. 

270.  Early  History  of  Electric  Railways.  —  The  application  of  elec- 
tric motors  which  probably  is  most  generally  known  and  appreciated  is 
in  propelling  the  electric  street  cars  that  are  to  be  found  in  nearly  every 
city  of  fair  size  in  this  country.  When  the  first  large  electric  railway 
enterprise  was  undertaken  in  the  year  1887  in  Richmond,  Va.,  prophe- 
cies of  failure  were  numerous,  and  the  discouragements  met  by  the  pro- 
moters of  the  enterprise  were  at  times  sufficient  to  dishearten  almost 
any  one.  Before  the  equipment  of  that  electric  railway  was  under- 
taken, various  experimental  electric  railways  had  been  laid  and  operated, 
and  several  had  been  actually  constructed  for  the  regular  carrying  of 
passengers,  but  none  of  them  were  of  such  magnitude  as  the  railway  at 


308  ELECTRICITY   AND   MAGNETISM 

Richmond,  and  none  served  to  prove  the  adaptability  of  electric  motors 
to  the  purpose  of  driving  cars  as  did  the  equipment  which  was  operated 
there. 

The  first  electric  railway  that  was  really  built  on  a  commercial  scale 
was  a  short  line  laid  in  Berlin,  Germany,  in  1879,  by  the  great  firm  of 
Siemens  and  Halske.  In  1883  the  first  electric  railway  opened  to  the 
public  in  the  United  States  was  operated  in  the  gallery  of  the  Chicago 
Railway  Exposition  on  a  track  about  1500  feet  long,  and  of  three  feet 
gauge.  This  electric  line  caused  a  great  stir  in  the  country  and  carried 
many  passengers  who  visited  the  Exposition.  The  motor  car  which  ran 
on  the  line  weighed  three  tons,  and  was  capable  of  running  at  a  speed 
of  nine  miles  an  hour.  It  was  therefore  quite  small  compared  even 
with  the  smallest  of  electric  street  cars  of  to-day,  which  weigh  from 
eight  to  twenty-five  tons  and  are  capable  of  running  at  speeds  of 
eighteen  to  twenty  miles  an  hour. 

Even  the  striking  though  modest  early  attempts  at  electric  railroading 
made  in  Berlin  and  Chicago  did  little  to  bring  electric  cars  into  general 
use,  though  they  did  serve  to  stir  up  the  interest  of  the  people.  The 
construction  of  the  early  motors,  as  viewed  to-day,  was  unmechanical 
and  inefficient,  so  that  great  improvements  were  required  before  the 
electric  cars  could  replace  horse  cars  or  cable  cars.  Since  1883 
the  electric  car  has  passed  through  a  period  of  marked  development 
both  in  this  country  and  in  Europe.  From  the  beginning  of  1883  until 
1888  several  small  electric  railways  were  put  into  operation  in  this  coun- 
try under  the  direction  of  Daft,  Van  Depoele,  Sprague,  and  others,  but 
until  the  latter  date,  by  which  time  Mr.  Sprague  had  made  the  Rich- 
mond road  a  success,  the  electric  car  cannot  be  said  to  have  proved 
itself  commercially  successful. 

From  1888  to  the  present  day,  electric  street  railways  have  grown  in 
number  and  in  favor  with  remarkable  rapidity.  So  much  is  this  true 
that  the  street-car  horse  has  been  banished  from  the  streets  of  nearly 
all  the  cities  of  the  country,  and  the  cable  railways  of  the  largest  cities 
are  also  rapidly  disappearing. 

271.  The  Principle  of  the  Electric  Railway.  —  The  principle  of  the 
electric  railway  is  very  well  illustrated  by  Figure  208.  In  this  figure,  A 
is  a  dynamo,  one  pole  of  which  is  connected,  through  a  switch  6"  and 


ELECTRIC   RAILWAYS 


309 


fuse  blocks  F,  to  the  street  railway  track,  and  the  other  pole  to  a  wire 
called  the  Trolley  Wire,  which  is  supported  over  the  track.  The  motor 
which  drives  the  car  is  placed  underneath  the  floor,  as  is  shown  at  M  in 
the  figure,  and  is  so  geared  to  the  axles  that  the  car  is  moved  along  by 
the  revolutions  of  its  armature.  In  order  that  current  may  be  supplied 


FIG.  208.  —  Diagram  illustrating  the  Fundamental  Features  of  the  Electric  Railway. 

to  the  motor,  a  movable  arm  extends  above  the  car,  and  presses  a  small 
wheel  against  the  trolley  wire.  This  arm  is  called  the  Trolley,  and  the 
current  is  conveyed  along  it  and  thence  down  to  the  motor.  After  the 
current  has  passed  through  the  motor,  it  completes  its  circuit  by  return- 
ing to  the  dynamo  through  the  rails  and  earth. 

272.  Railway  Motors.  —  The  motors  which  are  used  on  electric  cars 
are  series  wound,  and  their  speed  is  controlled  either  by  means  of  a  re- 
sistance that  is  placed  in  circuit  with  the  motor,  or  by  some  equivalent 
device.  The  motors  are  of  various  forms,  but  those  which  are  now 
commonly  used  are  completely  ironclad,  so  that  the  armature  is  pro- 
tected from  mechanical  injury,  or  from  being  splashed  by  water  from 
the  track. 

Nearly  all  of  the  street  railway  motors  that  are  now  used  are  arranged 
so  that  the  top  and  bottom  halves  of  the  ironclad  fields  may  be  easily 
separated  to  enable  repairs  to  be  made  to  the  armature  or  to  the  field 
coils.  This  is  an  important  point  to  the  electric  railway  owner,  because 
railway  service  is  very  hard  on  electric  motors.  The  machines  are  ex- 
posed to  dust  and  dirt,  and  are  often  forced  to  do  more  work  than  that 
for  which  they  were  designed.  On  account  of  the  cramped  space  which 
is  to  be  found  under  a  street  car,  the  motors  must  be  as  compact,  and  at 
the  same  time  as  light,  as  possible.  These  conditions  combine  to  make 


310  ELECTRICITY   AND   MAGNETISM 

repairs  frequent,  and  very  expensive  unless  the  various  parts  are  arranged 
so  that  they  may  be  easily  accessible.  A  railway  motor  is  illustrated  in 
Figure  136,  with  the  top  part  of  the  frame  thrown  back  so  that  the  arma- 
ture is  exposed. 

273.  Motor  Trucks.. —  The   axle  bearings  of  horse  cars  are  usually 
attached  directly  to  the  framework  of  the  car  floor,  and  the  same  thing 
is  done  in  cars  that  are  intended  to  be  drawn  after  electric  motor  cars  as 

Trailers  or  Tow  Cars.  Such  a 
construction  is  not  sufficiently 
substantial  in  electric  motor 
cars,  and  the  axle  bearings 
are  mounted  on  a  strong  iron 

r  i          i   •    i       •  11      i 

framework  which  is  called  a 

FIG.  209.  —  Electric  Railway  Truck  supporting  a     ^       .     /T^.  .         TT 

Motor  on  Each  Axle.  Truck  (Flg-  2O9)  •       Upon   the 

top  frame  of  this  truck  is  set 

the  car  body,  while  the  motors  are  usually  supported  from  the  axles  and 
the  truck  frame  work,  as  shown  in  the  figure.  The  figure  shows  one 
of  the  trucks  of  a  long  car  which  has  a  truck  at  either  end.  Short  cars 
have  only  one  truck. 

It  is  common  practice  to  place  two  motors  on  each  ordinary  motor 
car,  one  being  slung  on  each  axle.  This  is  done  so  as  to  use  as  fully  as 
possible  all  the  weight  of  the  car  in  giving  the  driving  wheels  a  grip  on 
the  rails.  When  one  motor  which  is  geared  to  only  one  axle  is  used,  the 
wheels  are  likely  to  slip  in  bad  weather  or  when  the  car  is  on  grades, 
and  the  speed  of  the  car  is  retarded,  or  its  progress  may  even  be 
stopped  altogether.  Some  inventors  have  arranged  gearing  so  that  one 
motor  may  drive  both  axles,  but  such  arrangements  have  never  proved 
successful  when  put  into  the  very  hard  service  to  which  the  electric  car 
is  subjected.  In  the  usual  construction,  where  the  motors  are  slung  from 
the  axles,  one  end  of  each  motor  is  supported  by  means  of  a  spring  fas- 
tened to  a  "  bolster  "  or  cross  timber  on  the  truck. 

274.  Railway  Pressure.  —  In  the  operation  of  electric  railway  motors 
we  have  the  overhead  trolley  wire  for  the  outgoing  electric  conductor,  and 
the  rails  furnish  a  path  for  the  returning  current.     An  electric  railway 
motor  is,  therefore,  in  an  electrical  position  which  is  entirely  similar  to 
that  of  an  ordinary  motor  that  is  arranged  to  be  moved  about,  and  the 


ELECTRIC   RAILWAYS  311 

lead  wires  of  which  are  slid  along  the  electric  mains.  Railway  motors 
used  in  this  country  are  all  designed  for  use  with  direct  currents,  and 
when  in  service  they  are  connected  in  parallel  across  constant  pressure 
circuits.  The  pressure  used  is  about  five  hundred  volts. 

Electric  railways  often  reach  out  so  far  from  the  power  station,  at 
which  the  electric  current  is  generated  that  a  lower  pressure  is  not  prac- 
ticable on  account  of  the  great  amount  of  copper  that  would  be  required 
to  carry  the  current  with  a  reasonable  loss  of  power.  On  the  other 
hand,  a  pressure  higher  than  five  or  six  hundred  volts  would  be  unsafe  to 
use  on  circuits  which  include  bare  wires  suspended  over  city  streets. 
The  pressure  of  five  hundred  volts  is  sufficient  to  give  a  severe  shock, 
but  it  is  not  ordinarily  dangerous  to  human  life,  as  has  been  proved  by 
long  experience,  though  horses  and  some  other  animals  which  are  more 
sensitive  to  electric  shocks  than  human  beings  have  been  killed  by  con- 
tact with  electric  railway  wires. 

As  the  lengths  of  electric  railways  have  been  increased,  it  has  some- 
times been  found  necessary  to  use  high  pressure  alternating  currents  for 
transmitting  the  power  from  the  central  generating  station.  The  press- 
ure is  then  reduced  by  stationary  transformers  located  at  sub-stations 
along  the  line,  where  the  current  is  also  rectified  by  means  of  rotary 
converters.1 

275.  Wiring  Requirements.  —  The  trolley  wire  of  an  electric  railway 
commonly  consists  of  a  conductor  of  hard  drawn  copper  of  from  No. 
o  to  ooo  B.  &  S.  gauge  in  size,  which  is  suspended  from  Span  Wires  or 


BOLTv 


JOINT  PLATE 


1 


Tl L HEAD  OF  RAIU 


RAIL         <  (CQ     ©      Q  Q     Q 


<^S  BONDS 

FiG.  210.  —  Rail  Joint  with  Joint  Plate  and  Bond  Wires. 

brackets  supported  on  poles.  When  the  distances  over  which  current 
must  be  carried  are  so  great  that  a  No.  ooo  wire  is  of  insufficient  con- 
ducting capacity,  feeders  may  be  run  from  the  power  station  to  various 
feeding  points,  where  they  are  connected  to  the  trolley  wire. 

1  Article  248. 


312  ELECTRICITY   AND   MAGNETISM 

The  conducting  capacity  of  the  track  must  also  be  carefully  looked 
after,  even  in  the  shortest  lines.  The  rails  of  which  the  track  is  com- 
posed are  about  thirty  feet  long,  and  their  ends  are  mechanically  con- 
nected by  means  of  joint  plates  or  Fish  Plates  and  bolts.  .On  account 
of  the  scale  which  is  found  on  the  rails  and  fish  plates,  the  joints  do  not 
conduct  electricity  satisfactorily,  and  it  is  necessary  to  join  the  rails  elec- 
trically as  well  as  mechanically.  For  this  purpose  what  are  called 

Bonds  are  used.     A  bond 
RAI1  .     .  — >    ,  is  a  short  piece  of  copper 

WIRE  \/BOND  \7  \    /WIRE  *•   ? 

/  \  /  \  /  \BQHP /__      wire,   the    ends   of  which 

BAIL  ,  ,  , 

are  riveted    into   the    ad- 

FlG.  211.  —  Electric  Railway  Track  reinforced  by  .        ,.  ., 

ContinuousWire.  joining  ends  of  two  rails, 

and  it  thus  serves  to  make 

a  good  electrical  connection  between  them  (Fig.  210).  Sometimes  a 
copper  or  an  iron  wire  is  placed  in  the  ground  between  the  rails,  and 
each  rail  is  connected  to  it  by  means  of  a  bond  (Fig.  211),  and  the 
electrical  connection  between  the  rails  is  made  by  means  of  this  con- 
tinuous wire. 

276.  Heavy  Railway  Service.  —  The  electric  motor  has  also  found  a 
place  in  railway  service  which  is  much  heavier  than  that  of  the  ordinary 
surface  street  railways. 

After  working  its  way  into  favor  on  street  railways,  it  came  rapidly  into 
use  upon  light  suburban  railways,  and  is  now  looked  upon  as  an  essential 
feature  of  any  new  system  of  city  rapid  transit.  Possibly  one  of  the  most 
striking  examples  of  the  use  of  electric  motors  upon  rapid  transit  sys- 
tems is  on  one  of  the  underground  railroads  in  the  city  of  London,  where 
electric  locomotives  are  used  to  draw  the  trains,  to  the  great  improve- 
ment of  the  atmosphere  and  cleanliness  of  the  tunnels.  The  equipment 
of  this  railway  was  followed  by  the  operation  of  an  elevated  railway  in 
Liverpool,  England,  and  the  Intramural  Railway  at  the  World's  Fair, 
both  of  which  were  started  in  the  spring  of  1893.  In  this  country  there 
are  now  in  operation  several  great  systems  of  elevated  and  city  rapid 
transit  electric  railways,  and  others  are  planned  in  which  electric  motors 
are  expected  to  play  a  prominent  part.  The  list  of  those  planned 
includes  the  great  underground  railroad  system  which  is  being  built  to 
give  the  inhabitants  of  New  York  City  a  satisfactory  means  of  transporta- 


ELECTRIC   RAILWAYS 


313 


tion  from  their  business  places  down  town  to  homes  located  a  number  of 
miles  away  to  the  north. 

Even  this  does  not  set  the  limit  to  the  field  of  the  electric  motor  when 
applied  to  railway  purposes.  The  heavy  passenger  trains  of  the  Balti- 
more and  Ohio  Railroad  are  drawn  by  means  of  electric  motors  through 
the  great  tunnel  under  the  city  of  Baltimore.  One  of  the  type  of  elec- 
tric locomotives  used  for  this  purpose  is  illustrated  in  Figure  212.  Fol- 
lowing this  example,  several  other  steam  roads  have  been  equipped  with 
electricity  for  short  distances,  and  the  results  have  proved  satisfactory. 
It  is  now  generally  believed  that  the  electric  car  will  invade  many  parts 
of  the  field  which  has  heretofore  been  exclusively  occupied  by  the  steam 


FIG.  212. —  Electric  Locomotive  for  hauling  Heavy  Trains. 

locomotive,  and  that,  in  some  kinds  of  service,  the  electric  motor  will  as 
completely  displace  steam  locomotives,  as  it  has  already  displaced  horse 
cars  and  cable  cars  for  surface  transportation  in  the  smaller  cities.  Ex- 
periments have  even  been  made  with  a  view  to  placing  electric  locomo- 
tives in  service  upon  main  trunk  railway  lines  ;  and  the  superintendent  of 
an  important  English  railway,  it  is  said,  believes  he  could  quickly  change 
his  whole  system  from  one  using  steam  locomotives  to  one  using  electric 
locomotives,  if  the  officers  of  the  road  so  directed.  Be  this  as  it  may,  the 
fact  is  plain  that  the  electric  motor  has  made  a  wonderful  record  for 
itself  when  used  upon  electric  railways  in  the  past  and  that  its  record 
will  be  much  more  remarkable  in  the  future. 

277.  Station  Management  and  Loads.  — The  question  of  getting  the 
greatest  possible  amount  of  work  out  of  his  machinery,  and  at  the  same 
time  of  expending  the  smallest  practicable  amount  of  money  for  its  safe 


ELECTRICITY   AND   MAGNETISM 


operation,  is  one  which  weighs  continually  on  the  mind  of  the  manager 
of  every  great  electric  plant.  It  is  this  which  leads  him  to  watch  all 
expenditures  and  keep  an  accurate  account  of  all  the  supplies  used  in 
his  station.  The  accounts  show  him  the  cost  of  fuel,  oil,  water,  labor, 
and  other  items,  for  every  1000  watts  generated  for  an  hour  by  the  dyna- 
mos. By  comparison  of  these  records  month  by  month,  and  also  the 
records  of  other  plants  of  similar  size,  it  is  possible  to  tell  whether  every 
economy  is  practised  which  will  not  cause  oppression  to  the  employees 
or  injury  to  the  plant. 

The  record  of  the  output  of  a  station  may  be  made  by  the  switchboard 
attendant,  who,  every  quarter  or  half  hour,  enters  the  readings  of  the 

feeder  amperemeters  and 
the  voltmeters  in  a  large 
book  which  is  properly 
ruled.  Sometimes  the  rec- 
ord is  made  by  integrating 
wattmeters1  and  other  au- 
tomatic instruments.  Fig- 
ure 213,  for  instance,  is  a 
reproduction  of  the  card 
taken  from  a  Recording 
Voltmeter  which  is  used  in 
a  large  central  station  for 
electric  lighting.  The  card 
shows  the  continuous  rec- 
ord of  the  pressure  which 
was  maintained  at  the  cen- 
tres of  distribution  during 
twenty-four  hours.  The  distance  between  two  successive  radial  lines 
represents  fifteen  minutes,  and  the  distance  along  the  radial  lines  in- 
cluded between  any  two  adjacent  circles  represents  two  volts.  Record- 
ing amperemeters  are  not  as  commonly  used  as  are  recording  voltmeters, 
as  the  voltmeter  record  is  a  check  upon  the  care  with  which  the  press- 
ure is  kept  constant,  while  there  is  no  particular  need  of  keeping  an 
extremely  exact  record  of  the  current. 

i  Article  188. 


FIG.  213.  —  Record  of  Bristol  Recording  Voltmeter. 


STATION   LOADS 


315 


\ 


A.M.  HOURS  P-M. 

FIG.  214. —  Load  Curve  of  Electric  Light  Station. 


Figure  214  shows  the  current  sent  out  from  a  certain  electric  light 
station  .during  twenty- four  hours.  The  hours  of  the  day  and  night  are 

laid  off  on  the  horizontal  line, 

and  the  current  at  any  hour 
is  equal  to  the  length  of  the 
corresponding  vertical  line 
which  is  included  between 
the  horizontal  line  and  the 
irregular  line.  This  shows 
very  plainly  the  effect  of  the 
dark  hours  of  the  afternoon 
in  causing  a  great  increase 
in  the  demand  for  light.  A 
curve  of  this  character,  which 
shows  the  current  sent  out 
from  a  station  at  every  mo- 
ment throughout  the  day,  is 
called  a  Load  Curve. 

The  total  amount  of  current  which  is  required  by  the  customers  of  an 
electric  light  plant  changes  from  hour  to  hour  with  comparative  slowness, 
as  is  shown  by  Figure  214,  and  such  an  amount  of  machinery  can  be 
kept  running  at  all  times  as  will  supply  the  load  most  economically.  A 
very  different  condition  exists  in  the  power  house  which  supplies  current 
to  electric  street  cars.  Figure  215  shows  the  amount  of  current  sent  out 
daring  one  hour  from  an  electric  railway  power  house,  the  record  being 
laid  out  in  the  same  way  as  that  of  Figure  214.-  This  figure  shows  the 
wonderful  range  and  rapidity  of  the  changes  in  the  current  supplied  by 
the  station.  Since  compound  wound  dynamos  which  keep  the  pressure 
fairly  constant  are  used  in  such  stations,  the  variations  of  the  current 
cause  similar  variations  of  the  load  on  the  dynamos  and  engines.  Every 
effort  has  been  made  to  reduce  the  range  of  these  changes  which  cause 
shocks  to  the  machinery  and  so  are  likely  to  finally  result  in  injury  or 
breakdown,  and  which  also  make  it  impossible  to  keep  the  machinery 
sufficiently  well  loaded  so  that  it  may  be  operated  with  the  greatest 
economy. 

One  method  which  is  used  with  a  view  to  decreasing  the  great  changes 


ELECTRICITY  AND    MAGNETISM 


in  the  load  on  railway  stations  calls  for  the  use  of  a  storage  battery. 
This  battery  has  its  positive  terminal  connected  directly  to  the  positive 
bus  bar  and  its  negative  terminal  to  the  negative  bus  bar ;  then  when  a 
great  demand  for  current  is  made  by  the  cars,  part  of  it  is  supplied  by 
the  battery,  and  the  dynamos  and  engines  are  relieved  to  some  extent. 
When  the  current  required  by  the  cars  is  small,  the  battery  takes  cur- 
rent from  the  dynamos,  by  which  means  it  is  kept  charged  ;  and  thus 
the  variations  of  the  load  on  the  engines  are  made  much  smaller  than 
they  would  be  without  the  battery.  Storage  batteries  are  also  used  in  a 


10    15    20    25   30    35   40    45    CO    55    60 


Fig.  215.  —  Load  Curve  taken  for  One  Hour  in  Electric  Railway  Plant. 

few  large  American  and  various  foreign  electric  light  stations  to  aid  in 
supplying  the  current  during  the  period  of  greatest  load,  and  the  bat- 
teries are  then  recharged  during  the  period  of  light  load.  They  cannot 
be  very  satisfactorily  used  in  small  plants  because  of  their  great  expense. 
278.  Electric  Car  Controlling.  —  The  detail  improvements  which  have 
had  the  greatest  effect  upon  the  loads  of  electric  railway  power  stations 
lie  in  the  street-car  motors  and  the  manner  in  which  they  are  controlled. 
The  earlier  motors  which  were  put  upon  street  cars  were  wired  up  so 
that  the  two  machines  were  put  permanently  in  parallel,  and  they  were 
then  controlled  by  means  of  resistances  put  in  series  with  them.  Some 
cars  are  still  controlled  in  this  manner.  When  the  car  is  to  be  started, 


ELECTRIC   RAILWAY  MOTORS  317 

a  controller  lever  is  moved  so  that  it  connects  the  two  motors  to  the 
circuit  in  parallel  with  each  other  and  in  series  with  a  resistance.  To 
make  the  cars  run  faster,  the  resistance  is  gradually  cut  out  of  the  cir- 
cuit, and  finally  a  certain  portion  of  the  series  field  coils  of  the  motors 
may  be  cut  out  also,  if  a  particularly  high  speed  is  desired. 

Another  way  of  controlling  the  speed  of  street  cars  is  by  what  is  called 
the  "  commutated  field  "  method.  In  this  case  the  fields  of  the  motors 
are  wound  in  separate  divisions,  usually  three  in  number,  and  the  speed 
of  the  motor  is  controlled  by  connecting  the  field  coils  of  each  motor  in 
different  combinations.  When  a  car  is  to  be  started,  the  switch  handle 
is  moved  to  a  position  which  causes  the  three  field  coils  on  each  motor  to 
be  connected  in  series  with  the  armature.  To  cause  the  car  to  run 
faster,  the  lever  is  moved  from  point  to  point,  commutating  the  fields 
into  various  arrangements,  until  on  the  seventh  and  last  notch  the  indi- 
vidual field  coils  are  in  parallel.  It  is  of  some  interest  to  know  that  this 
system  was  developed  by  Sprague  while  working  on  the  historic  line  in 
Richmond.  These  methods  of  motor  control  are  also  used  to  some 
extent  for  hoisting  and  other  variable  speed  series-wound  motors. 

279.  Series-parallel  Control.  —  Both  of  the  earlier  forms  of  controllers 
served  very  well  as  far  as  the  handling  of  cars  is  concerned,  but  the  use 
of  resistances  in  the  manner  described  causes  a  great  waste  of  power, 
and  consequently  the  cars  require  a  great  deal  of  current  in  starting. 
This  in  turn  has  an  effect  in  increasing  the  suddenness  and  magnitude 
of  the  changes  of  load  at  the  power  station.  The  need  for  a  more 
efficient  controller  which  would  waste  less  power  and  allow  the  cars  to 
start  with  less  current  became  so  pressing  that  various  devices  were 
designed  to  meet  the  want.  All  of  these  were  reduced  to  some  form  of 
"  series-parallel "  controller. 

The  pull  or  Torque  with  which  a  series-wound  motor  tends  to  start 
depends  only  upon  the  current  flowing  through  it.  If  two  motors  are 
connected  in  parallel,  and  enough  current  is  passed  through  them  to 
start  a  street  car,  the  total  amount  of  current  may  be  as  much  as  eighty 
amperes.  The  starting  effort  in  this  case  is  caused  by  forty  amperes 
flowing  through  each  motor.  Now,  if  the  same  two  motors  are  con- 
nected in  series  with  each  other,  and  a  current  of  forty  amperes  is  per- 
mitted to  flow  through  them,  each  will  exert  the  same  starting  effort  as 


ELECTRICITY   AND    MAGNETISM 


before,  and  the  car  will  start  with  the  expenditure  of  only  half  the  cur- 
rent. Having  started  the  car,  the  motors  must  be  connected  in  parallel 
in  order  that  they  may  run  at  a  reasonably  high  speed,  because  when 
the  motors  are  in  series  the  total  pressure  of  500  volts  is  divided  between 
them,  and  each,  therefore,  gets  only  about  250  volts.  The  speed  of  a 
motor  depends  directly  upon  the  pressure  at  its  armature  terminals,  and 
therefore,  when  connected  in  series,  the  motors  will  run  at  only  half 
speed. 

280.    Starting  Current  Curves.  —  The  actual  process  of  controlling  a 
car  by  the  series-parallel  method  consists  of  starting  the  car  with  the 

motors  in  series  with  each 
other  and  a  resistance,  cut- 
ting the  resistance  out  of 
circuit,  and  then  by  a  series 
of  commutations  indicated 
in  Figure  216,  putting  the 
motors  in  parallel  with  each 
other,  and  in  series  with  the 
resistance.  This  resistance 
is  finally  cut  out  of  the  cir- 
cuit to  make  the  car  run  at 
a  high  speed. 

The  comparative  effi- 
ciency of  operating  cars 
with  motors  equipped  with 

rheostat    and   with    series- 
FlG.  216.  —  Diagrammatic  Illustration  of  the  Opera-  n          .      .„ 

tion  of  a  "Series-parallel  Controller."  Paralld  controllers  IS   illus- 

trated in  Figure  217.     The 

time  after  current  is  admitted  to  the  motors  is  laid  off  on  the  hori- 
zontal line,  and  the  distance  from  the  horizontal  to  the  wavy  lines  at 
any  point  shows  the  amount  of  current  flowing  through  the  motors 
at  that  instant.  The  upper  wavy  line  shows  the  current  consumed 
when  a  certain  pair  of  motors  were  controlled  by  a  rheostat,  and  the 
lower  wavy  line  shows  the  current  consumed  when  the  same  motors 
were  controlled  in  the  series-parallel  fashion.  During  the  first  ten  sec- 
onds the  rheostat  control  required  twice  as  much  current  on  the  average 


'ARM.    NO.  2 

"FIELD  NO.  2 
GROUND 


OTHER   USES   OF   ELECTRIC   MOTORS 


319 


as  did  the  series-parallel  control,  and  during  the  first  eighteen  seconds 
the  rheostat  required  one-half  more  current. 

A  similar  figure  might  also  be  drawn  to  illustrate  the  difference  between 
the  amounts  of  current  used  by  a  careful  motorman  in  starting  his  car 
and  by  a  careless  man. 
The  former  always  moves 
his  controller  lever  from 
point  to  point  with  care, 
and  permits  the  motors 
to  gather  speed  before 
passing  from  one  point 


/    . 

\ 

STARTING  CL 
SHOWING 
VARIATION  OF  CU 

WITH 

R 
RR 

v'E 
ENT 
STATIC 

Sfe 

1 

y 

I 

1 

\ 

CONTROL 

1 

/ 

\ 

QU  PPEDW1TH  TW03-E.,    800 

\ 

, 

s 

N 

\ 

/ 

\ 

/ 

\ 

i 

/ 

S 

7 

^ 

b 

1 

l 

•  — 

•~~_ 

\ 

'\ 

/I 

N 

/ 

K 

A/ 

M 

A 

1 

E3 

M 

1! 

e. 

M 

AVI 

,3 

A' 

u 

1 

r; 

.:. 

HI 

T^ 

2.4- 

SECONDS 

FIG.  217.  —  Illustration  of  the  Saving  effected  by  the  Use 
of  the  "  Series-parallel  Controller." 


to  a  higher  one.  By 
neglecting  this  precau- 
tion a  considerably  larg- 
er current  may  be  used 
than  is  necessary.  Some 
steam  railroads  pay  a 
bonus  to  the  engine 
driver  who  succeeds  in 
making  his  runs  each 

month  with  the  least  coal,  and  it  would  be  a  paying  investment  for 
many  electric  railroads  to  pay  a  bonus  to  their  motormen  who  succeed 
in  making  the  runs  with  the  least  current. 

281.  A  Few  of  the  Uses  of  Stationary  Electric  Motors.  —  During  the 
past  decade  electric  motors  have  come  to  be  almost  a  necessity  to  people 
who  live  in  small  cities,  and  use  small  amounts  of  power.  The  wonder- 
ful way  in  which  electric  motors  have  come  into  general  use  is  very  strik- 
ing. The  number  used  in  Chicago  in  the  year  1889  was  very  small, 
while  in  1894  motors  to  more  than  four  thousand  horse-power  capacity 
\vere  supplied  with  current  from  the  distributing  system  of  the  Edison 
Illuminating  Company  of  that  city.  The  increase  since  then  has  been 
equally  rapid.  In  addition  to  these  motors,  many  more  are  supplied 
with  current  from  either  central  or  isolated  plants.  Chicago  is  not  at  all 
exceptional  in  the  number  of  electric  motors  which  its  inhabitants  use, 
for  large  numbers  are  also  used  in  New  York,  Boston,  Philadelphia,  and 
other  large  cities.  In  fact,  electric  motors  are  as  necessary  to  the  small 


320 


ELECTRICITY   AND   MAGNETISM 


users  of  power  who  live  in  American  cities,  large  or  small,  as  gas  engines 
are  to  the  citizens  of  Paris,  and  they  have  also  become  household  neces- 
sities in  many  places. 


FIG.  218.  —  Large  Fan  driven  by 
Electric  Motor. 


FIG.  219.  — Small  Fan 
Motor  and  Fan. 


The  use  of  electric  motors  in  small  shops  and  for  household  purposes 
is  by  no  means  limited  to  the  large  cities  ;  but  in  all  places  where  a  power 
supply  is  at  hand  throughout  the  day,  the  motors  are  found  in  many 


FlG.  220.  —  Great  Organ  Bellows  driven  by  Electric  Motor. 


kinds  of  service.  One  of  their  commonest  uses  is  to  drive  small  fans  for 
stirring  up  the  air  in  a  room  in  the  hot  summer  days.  Such  Fan  Motors 
are  common  in  offices,  theatres,  and  public  places.  An  interesting  use 


OTHER  USES  OF  ELECTRIC   MOTORS 


321 


of  fan  motors  is  made  on  the  electrically  lighted  trains  of  the  Pennsyl- 
vania and  other  railroads,  the  dining  cars  of  which  are  made  very  com- 
fortable on  hot  summer  evenings  by  several  fan  motors,  which  take 
current  from  the  electric  light  circuits. 

An  example  of  motors  used  with  an  isolated 
plant  is  to  be  seen  in  the  great  plant  of  the 
Auditorium  Hotel  and  Theatre  in  Chicago,  where 
motors  having  a  combined  capacity  of  several 
hundred  horse  power  are  in  daily  use.  These 


FIG.  221.  —  Sewing  Machine  with  Electric  Motor. 


FIG.  222.  —  Dentist's  Lathe 
with  Electric  Motor. 


motors  are  used  to  drive  ventilating  fans  and  small  blowers,  as  shown 
in  Figures  218  and  219,  to  run  coal  and  ash  hoisters,  meat  choppers 
and  coffee  grinders  for  the  kitchen,  machinists'  tools  for  the  repair 
shop,  bellows  for  the  great  organ  (Fig.  220),  to  drive  a  small  dynamo 
which  furnishes  current  for  the  hotel  bells,  and  for  other  purposes. 
Some  of  the  dynamos  of  this  plant  are  required  to  run  all  day  and  all 
Y 


322 


ELECTRICITY   AND    MAGNETISM 


night,  so  that  a  supply  of  current  is  always  on  hand  by  means  of  which 

the  motors  may  be  operated. 

The  uses  to  which  electric  motors  may  be  put  are  almost  endless,  but 

a  few  of  the  common  applications  are  illustrated  in  the  figures  of  this 

article.  In  Figures  221 
and  222  are  shown  a 
sewing  machine  and  a 
dentist's  lathe,  each 
with  a  motor  con- 
nected to  it.  Figure 
223  shows  how  a  pump 
may  be  driven  by  an 


FIG.  223.  —  Electric  Pump. 


electric     motor.        An 


electric  motor  driving  a  contractor's  hoist,  which  is  used  in  the  con- 
struction of  a  large  building,  is  shown  in  Figure  224,  and  a  small  elec- 
tric mining  hoist  is  shown  in  Figure  225. 

This  list  of  illustrations  might  be  extended  to  an  indefinite  extent 
without  exhausting  the  various  purposes  for  which  electric  motors  may 
be  used,  and  for  which,  indeed,  they  are  used  in  great  numbers. 

282.  Counter  Electric  Pressure  of  a  Motor.  —  It  has  already  been 
explained  that  the  cutting  of  lines  of  magnetic  force  by  a  conductor 
always  sets  up  an  electromotive  force  in  the  conductor.1  This  occurs  in 
a  dynamo  armature  when  the  conductors  are  caused,  by  the  rotating  of 
the  armature,  to  move  across  the  lines  of  force  of  the  magnetic  field  in 
front  of  the  pole  pieces.  And  the  electrical  pressure  thus  produced  in 
the  dynamo  armature  causes  a  current  to  flow  when  the  circuit  is  com- 
plete. A  similar  pressure  is  also  set  up  in  the  armature  conductors  of 
an  electric  motor,  when  they  are  caused  to  cut  the  lines  of  force  of  the 
magnetic  field  by  the  rotation  of  the  armature  ;  but  this  pressure  pro- 
duced in  the  motor  armature  is  opposed  to  the  current  which  causes  the 
rotation  of  the  armature,  and  it  is  therefore  called  a  Counter  Electric 
Pressure,  or  Counter  Electromotive  Force.2 

When  a  motor  armature  gathers  speed  after  it  is  started,  the  counter 
electric  pressure  becomes  greater  and  greater  as  the  speed  grows  faster 
and  faster,  because  more  lines  of  force  are  cut  in  each  second ;  and  the 

1  Article  195.  2  Article  207. 


OPERATION   OF   ELECTRIC   MOTORS 


323 


current  flowing  through  the  armature  meets  the  increased  opposition  of 
this  counter  electric  pressure.  The  speed  and  counter  pressure  of  the 
armature  increase  until  the  armature  has  reached  a  speed  where  the 
counter  electric  pressure  is  nearly  equal  to  the  pressure  of  the  circuit  to 

which  the  motor  is  connected.  Then 
the  speed  and  current  become  con- 
stant, and  remain  so  as  long  as  the 
load  on  the  motor  and  the  circuit 
pressure  remain  unchanged. 

283.  Effect  of  Armature  Resistance. 
—  The  counter  pressure  of  the  motor 
armature  can  never  become  equal  to 
the  pressure  of  the  circuit,  because  a 
certain  pressure  is  required  to  push  the 
current  through  the  resistance  of  the 
armature  conductors.  In  shunt-wound 
motors  the  sum  of  this  with  the  counter 
pressure  is  equal  to  the  circuit  pressure. 


FIG.   224.  —  Hoisting  Derrick  with 
Electric  Motor. 


FIG.  225.  —  Small  Electric  Mining  Hoist. 


When  the  current  flowing  through  the  motor  is  multiplied  by  the 
circuit  pressure,  the  product  gives  the  amount  of  power  (in  watts)  which 
is  given  to  the  motor  by  the  circuit. 

The  product  of  the  armature  current  by  the  pressure  which  is  required 
to  push  the  current  through  the  armature  conductors  of  the  motor  gives 


324 


ELECTRICITY   AND   MAGNETISM 


the  watts  wasted  in  the  heating  of  the  armature  conductors.     This  is 
called  the  C*R  loss  of  the  armature,  and  should  be  quite  small,  in  order 

that  the  motor  may  be  reason- 
ably efficient  and  regulate  well. 
The  product  of  the  armature 
current  with  the  counter  elec- 
tric pressure  is  equal  to  the 
power  that  is  exerted  in  caus- 
ing the  armature  to  rotate; 
and  the  more  nearly  the  counter 
pressure  approaches  the  circuit 
pressure  when  the  motor  is  run- 
ning, the  better  is  the  efficiency 
of  the  motor  and  the  better 
will  it  regulate.  We  sometimes 
hear  of  an  attempt  to  build  an 
improved  motor  which  pro- 
duces no  counter  electromotive 
force ;  but  all  such  attempts 
are  doomed  to  failure,  since 
the  success  of  the  motor  depends 
upon  its  producing  the  counter 
pressure. 

284.    Starting  Box  or  Rheo- 
stat.—  As  the  resistance  of  the 
armature  of  an  electric  motor 
is  usually  quite  small,  for  the 
reasons    presented    in    Article 
283,  some  special  means  must 
be  provided  for  avoiding   too 
great  a  rush  of  current,  when 
a   motor   is  to   be  started    by 
connecting  it  to  a  circuit. 
The  resistance  of  the  armature  of  a  ten  horse  power  motor,  which  is 
designed  to  be  operated  in  connection  with  a  2  20- volt  circuit,  may  be 
about  three-tenths  of  an  ohm,  or  even  less.     The  current  that  such  an 


FIG.  2250.  —  Exterior  and  Interior  of 
Motor  Starting  Box. 


OPERATION   OF   ELECTRIC   MOTORS 


325 


armature  may  be  expected  to  carry  at  full  load  is  nearly  40  amperes. 
Now,  if  this  armature  is  connected  across  the  22O-volt  circuit  while  it  is 
at  rest,  the  only  opposition  which  the  current  meets  is  that  caused  by 
the  resistance,  since  no  counter  electric  pressure  is  developed  while  the 


STARTING  BOX 


•MAIN  SWITCH 


SHUNT-WOUND  MOTOR 

FlG.  225^. —  Diagram  of  Plain  Starting  Box  connected  in  Circuit  with  Shunt-wound  Motor. 

armature  is  standing  still;  and  the  current  that  instantly  tends  to  flow 
through  the  armature  is,  in  accordance  with  Ohm's  Law,  22O/.3  =  733  -f 
amperes,  —  a  sufficient  current  to  overheat  and  ruin  the  armature,  un- 
less the  circuit  is  instantly  broken. 

It  is  necessary,  therefore,  to  insert  a  certain  amount  of  resistance  in 
series  with  the  armature,  when  the  motor  is  to  be  started,  so  that  the 


326  ELECTRICITY   AND   MAGNETISM 

current  may  be  choked  back  until  the  rotation  of  the  armature  has 
reached  a  speed  which  causes  it  to  produce  sufficient  counter  pressure 
to  prevent  an  excessive  rush  of  current  through  the  windings.  A  resist- 
ance box  made  up  for  this  purpose  is  called  a  Starting  Box  or  Starting 
Rheostat.  A  usual  form  is  illustrated  in  Figure  2250.  It  consists  of 
a  box  full  of  iron  wire  resistance  coils,  wound  on  asbestos  tubes  or 
some  similar  incombustible  material.  These  are  all  connected  in  series, 
and  one  end  of  the  series  is  connected  to  a  binding  post.  The  resist- 
ance coils  are  connected  at  intervals  to  buttons  that  are  shown  on  the 
front  of  the  box.  The  lever  arm  shown  in  the  figure  is  connected  to  a 
binding  post  by  a  short  wire  behind  the  box  cover. 

When  the  starting  box  is  connected  in  the  circuit  with  a  shunt-wound 
motor  in  accordance  with  the  illustration  (Fig.  225^),  the  motor  may 
be  safely  started  by  the  following  procedure.  When  the  main  switch 
is  closed,  the  current  flows  through  the  field  winding  and  sets  up  the 
field  magnetism.  The  handle  of  the  box  lever  L  is  then  moved 
slowly  toward  the  right,  from  button  to  button.  When  the  lever  con- 
tact is  on  the  button  marked  a,  all  the  resistance  of  the  starting  box 
is  connected  in  series  with  the  motor  armature.  As  the  lever  contact 
moves  toward  the  right,  the  resistance  included  in  the  circuit  is  gradu- 
ally reduced ;  and  when  the  lever  contact  stands  on  the  button  marked 
"  on,"  all  of  the  resistance  has  been  cut  out,  and  the  motor  armature  is 
connected  directly  to  the  circuit.  In  the  meantime  the  armature  has 
been  given  an  opportunity  to  come  up  to  its  regular  speed,  so  that  the 
counter  electromotive  force  is  amply  sufficient  to  prevent  the  current 
from  becoming  excessive. 

The  resistance  of  the  coils  in  the  starting  box  must  be  sufficient  to  pre- 
vent much  more  than  the  normal  current  of  full  load  from  flowing  through 
the  armature  when  it  is  stationary.  In  the  case  of  the  ten  horse  power 
motor  referred  to  above,  the  starting  box  should  contain  not  less  than 
4  or  5  ohms,  and  the  rheostat  coils  should  be  capable  of  carrying  the 
full  load  current  for  several  minutes  without  becoming  dangerously  hot. 

285.  Automatic  Release.  —  Most  starting  boxes  nowadays  are  fitted 
with  an  automatic  arrangement  that  allows  the  contact  lever  to  return  to 
its  "  off "  position  in  case  the  main  switch  is  opened  or  the  current 
supply  fails  for  any  reason.  This  is  very  desirable,  since  it  disconnects 


OPERATION   OF   ELECTRIC   MOTORS 


327 


the  motor  from  the  circuit,  and  prevents  it  from  being  injured  in  case  the 
current  supply  is  renewed  while  the  armature  is  at  rest.  This  construc- 
tion also  makes  it  impossible,  after  the  motor  is  started,  to  leave  the 
contact  lever  permanently  in  a'mid-position,  which  might  result  in  dan- 
gerously overheating 
the  resistance  coils, 
and  such  starting  boxes 
are  recommended  in 
the  rules  of  the  Un- 
derwriters, and  are 
required  for  many  lo- 
cations. Figure  225  c 
shows  the  circuit  con- 
nections of  a  box  of 
this  character. 

Some  starting  boxes 
also  have  an  overload 
cut-off  arrangement, 
which  stops  the  motor 
if  it  is  overloaded. 

286.  Starting  and 
Stopping  Motors.  — 
When  a  shunt-wound 
motor  is  to  be  started, 
the  main  switch  is  first 
closed.  The  contact 
lever  of  the  starting 
box  is  moved  to  the 
first  button,  the  field  magnets  become  excited,  and  the  motor  armature 
begins  to  move.  The  contact  lever  is  then  slowly  moved  to  the  "  on  " 
button,  while  the  motor  gathers  speed.  Care  must  be  taken  to  avoid 
cutting  the  resistance  of  the  starting  box  out  of  the  circuit  too  rapidly, 
in  order  that  the  starting  current  may  not  be  too  great ;  but  the  lever 
must  not  be  permitted  to  stand  more  than  a  short  time  (perhaps  a  half 
minute)  on  any  one  button,  because  the  resistance  coils  are  intended  to 
carry  a  large  current  for  only  a  short  time. 


SHUNT-WOUND  MOTOR 

FIG.  225  c.  —  Diagram  of  Automatic  Starting  Box  connected 
in  Circuit  with  Shunt-wound  Motor. 


328 


ELECTRICITY   AND    MAGNETISM 


In  stopping  a  motor,  the  main  switch  should  be  opened.  After  the 
armature  has  nearly  stopped,  the  contact  lever  of  an  automatic  starting 
box  returns  to  its  "  off"  position.  If  the  starting  box  is  not  automatic, 
the  lever  must  be  returned  by  hand. 

287.  Reversing  the  Direction  of  Rotation. — To  reverse  the  direction 
of  the  rotation  of  a  motor  armature,  it  is  necessary  to  reverse  the  relative 
direction  in  which  the  current  in  the  conductors  flows  through  the  mag- 
netic field.  Consequently,  either  the  current  in  the  armature  coils  must 
have  its  direction  reversed  while  the  field  magnets  retain  their  original 
polarity,  or  else  the  polarity  of  the  field  magnets  must  be  reversed  vvith- 


Fir,.  225  d.  —  Reversing  Switch. 

out  changing  the  direction  of  the  current  in  the  armature.  A  simple 
exchange  of  the  positive  and  negative  terminal  connections  of  the  main 
circuit  to  the  motor  has  no  effect  on  the  direction  of  the  rotation  of  the 
armature,  since  the  polarity  of  the  fields  is  reversed  by  this  exchange  at 
the  same  time  that  the  direction  of  the  current  in  the  armature  coils  is 
reversed.  This  exchange,  therefore,  does  not  change  the  relative  con- 
ditions. Consequently,  to  effect  a  reversal  of  rotation,  change  either  the 
direction  of  the  current  in  the  field  coils,  or  the  direction  of  the  current  in 
the  armature,  but  not  both.  Reversing  the  direction  of  the  current  in 
the  armature  coils  is  the  commoner  method  of  effecting  the  reversal  of 
rotation.  If  a  motor  is  to  be  often  reversed,  as  in  elevator  work,  it  is 
usual  to  provide  it  with  relatively  powerful  field  magnets,  so  that  the 


ELECTRIC   POWER  IN   FACTORIES 


329 


positions  of  the  brushes  on  the  commutator  need  not  be  changed  during 
its  ordinary  operation.  Figure  225^  illustrates  a  form  of  reversing 
switch  which  is  often  used  with  electric  motors. 

288.  Motors  and  Manufactories.  —  A  place  in  which  electric  motors 
are  coming  to  be  very  much  appreciated  and  widely  used  is  in  large 
manufactories.  The  ordinary  method  of  carrying  power  through  shops 
by  means  of  great  belts  and  heavy  shafts  is  very  wasteful  of  power.  The 
attached  table,  taken  from  one  in  Professor  Flather's  book  on  power  meas- 
urements, shows  the  amount  of  power  lost  in  belting  and  shafting,  and 


FIG.   226.  —  Machine   Shop   in   which  the 
Machinery  is  driven  by  Belts. 


FIG.  227.  —  Machine   Shop   in  which  the 
Machinery  is  driven  by  Electric  Motors. 


the  amount  actually  delivered  where  it  is  required  for  use,  for  every  hun- 
dred horse  power  developed  by  the  engine.  The  table  shows  that  from 
one-fourth  to  four-fifths  of  the  power  of  the  engines  is  actually  wasted 
in  simply  making  shafting  revolve,  and  causing  the  belts  and  gears  to  run. 


NAME  OF  WORKS 

POWER  LOST, 
PER  CENT 

POWER  USED, 
PER  CENT 

Union  Iron  Works        ....... 

23 

77 

Frontier  Iron  and  Brass  Works    ..... 

32 

68 

Baldwin  Locomotive  Works          .                   ... 

80 

20 

Wm.  Sellers  and  Company  ...... 

40 

60 

Pond  Machine  Tool  Company      ..... 

41 

59 

Bridgeport  Forge  Company          

5° 

50 

Yale  and  Towne  Company  .... 

49 

51 

Ferracute  Machine  Company        ..... 

3i 

69 

330 


ELECTRICITY   AND    MAGNETISM 


Shafts  and  belts  are  a  great  nuisance  in  shops,  and  any  convenient 
arrangement  which  can  take  their  place  would  be  very  useful,  even  if  it 
did  not  save  power.  A  convenient  arrangement  which  takes  their  place 
and  at  the  same  time  saves  much  power  is  of  the  greatest  service.  It  is  in 
this  place  that  the  electric  motor  shows  one  of  its  finest  characteristics. 
In  Figure  226  is  shown  a  large  machine  shop  in  which  the  power  is  dis- 
tributed by  shafts  and  belts,  which  give  the  shop  somewhat  the  appear- 
ance of  a  forest,  while  in  Figure  227  is  shown  a  similar  shop  after  the 


FlG.  228.  —  Machinist's  Lathe  arranged  to  be  driven  by  Electric  Motor. 

lathes,  planers,  and  other  machines  are  arranged  to  be  driven  by  electric 
motors.  The  motors  are  close  to  the  machines,  and  the  electric  \ures 
leading  to  them  are  put  out  of  the  way  so  that  the  shop  presents  a  much 
improved  appearance. 

The  improvement  is  as  great  in  fact  as  in  appearance,  because  the 
removal  of  shafting  and  belts  removes  a  great  source  of  danger  and 
inconvenience,  and  electrical  distribution  of  the  power  is  much  less 
wasteful  than  distribution  by  shafts  and  belts.  A  properly  arranged  elec- 
trical distribution  of  power  also  makes  it  possible  to  place  every  lathe, 


ELECTRIC   POWER  IN   FACTORIES  331 

planer,  or  other  machine  at  the  position  in  the  shop  where  it  may  be 
used  most  conveniently ;  and  the  speed  of  each  machine  may  be  ad- 
justed by  the  turn  of  a  hand  to  best  suit  any  work  that  is  being  done. 
These  conditions  enable  an  electrically  driven  factory  to  execute  more 
work  than  a  similar  establishment  where  belt  driving  is  used.  The  ad- 
vantages which  may  be  gained  by  using  electricity  instead  of  belts  and 
shafts  is  worth  a  great  many -dollars  to  the  owners  of  the  shops;  and 
many  shops  have,  therefore,  been  arranged  for  electrical  transmission, 
while  many  more  are  being  so  arranged.  Figure  228  shows  the  way  in 
which  a  motor  may  be  applied  to  drive  a  lathe  or  drill  or  other  machine, 
while  Figure  229  shows  a  large  travelling  crane  which  is  driven  by  elec- 
tric motors.  Such  cranes  are  used  in  nearly  all  large  machine  shops. 


FIG.  229. — Travelling  Crane  with  Electric  Motors. 

The  arrangement  of  electric  motors  which  will  give  the  best  results  in 
any  shop  depends  upon  a  great  many  things,  and  can  only  be  arrived  at 
by  good  judgment.  The  ideal  method  would  be  to  have  one  or  more 
motors  built  as  a  part  of  every  machine  in  the  establishment,  but  this 
would  make  the  machinery  too  costly,  and  consequently  cannot  be 
carried  out,  though  it  would  probably  be  the  most  convenient  and  sat- 
isfactory arrangement  which  it  is  possible  to  make.  The  next  best 
arrangement,  and  the  one  which  is  usually  adopted,  is  to  have  all  large 
machines  which  require  considerable  power  furnished  with  separate 
motors.  These  may  be  built  into  the  machines,  thus  doing  away  with  all 
unnecessary  belting  or  gearing,  or  they  may  be  directly  belted  to  the 
usual  driving  pulleys  of  the  machines.  Smaller  machinery  may  be 
arranged  in  groups  of  two  to  six  machines  with  a  motor  to  supply  power 
to  the  machines  of  each  group  through  a  light  shaft. 


332 


ELECTRICITY   AND    MAGNETISM 


The  amount  of  power  required  to  drive  different  classes  of  machinery 
is,  as  a  general  rule,  quite  uncertain. 

The  width  of  the  belt  which  is  commonly  used  on  a  machine  is  some 
indication  of  the  power  required,  as  it  may  be  assumed  that  a  single 
leather  belt  when  running  at  the  ordinary  speed  used  in  shops  will  satis- 
factorily drive  from  one  to  two  horse  power  per  inch  of  width.  A  double 
belt  will  generally  drive  about  twice  as  much  as  a  single  one.  An  exact 
estimate  of  the  power  used  by  any  machine  cannot  be  made  from  the 
size  of  its  belt,  however,  since  the  driving  power  of  a  belt  depends, 
amongst  other  conditions,  directly  upon  its  speed,  and  even  at  ordinary 

speeds  it  may 
transmit  very 
much  more 
power  than  the 
rule  given  above 
would  indicate, 
though  its  oper- 
ation would  be 
unsatisfactory. 

289.  Electric 
Launches  and 
Automobiles.— 
Before  leaving 
this  subject,  the 
use  of  electricity 
upon  boats  and 
auto  mobiles 
must  be  touched 

upon.  Figure  230  shows  one  of  the  "  electric  launches  "  similar  to  those 
which  proved  such  a  success  on  the  lagoons  at  the  Chicago  World's 
Fair,  and  which  are  now  built  for  general  sale.  These  boats  are  very 
much  like  small  steam  or  gasoline  launches,  but  instead  of  a  hot  steam 
boiler  and  engine,  or  a  disagreeable  gasoline  engine,  an  electric  motor, 
which  may  be  put  out  of  sight  under  the  floor,  is  operated  by  electric 
current  from  a  storage  battery,  the  cells  of  which  are  placed  under  the 
seats  and  floor  so  as  to  act  as  ballast.  The  boat  is  not  so  independent 


FIG.  230.  —  Electric 
Launch. 


OTHER   USES   OF   ELECTRIC   MOTORS 


333 


STORAGE  BATTE 


as  a  steam  or  gasoline  launch,  as  the  battery  must  be  charged  every  day  to 
keep  it  in  good  order  for  operating,  but  electric  launches  are  convenient 
for  use  wherever  current  can  be  obtained  for  charging  the  batteries. 

During  the  past  two  or  three  years  the  automobile  has  been  so  far 
developed  that  it  has  become  a  familiar  sight  on  the  streets  of  most  of 
our  cities  and  towns.  These  horseless  carriages  are  driven  by  steam, 
gasoline,  and  electricity.  The  latter  method  is  popular  in  the  large 
cities  where  electric 
current  may  be  readily 
obtained  for  charging 
the  storage  batteries, 
and  it  has  given  reason- 
ably satisfactory  results. 
The  motor  of  from  one- 
half  to  two  or  more 
horse  power  is  usually 
geared  to  one  of  the 
axles  of  the  carriage 
and  is  supplied  with 
power  from  a  storage 
battery  stored  neatly 
under  the  seat.  The 
motor  is  controlled  by 

a  rheostat,  the  handle  of  which  is  within  easy  reach  of  the  driver.  Such 
vehicles,  one  of  which  is  exhibited  in  Figure  231,  have  not  yet  proved 
themselves  to  be  so  efficient  that  they  seem  likely  to  drive  all  the  cab 
and  dray  horses  from  our  city  streets,  after  the  manner  of  the  electric 
railway,  which,  within  the  space  of  ten  years,  caused  the  street-car  horse 
to  become  an  oddity. 

QUESTIONS 

1.  How  long  has  it  been  since  electric  lighting  plants  became  common  ? 

2.  About  when  was  the  first  central  station  put  in  operation? 

3.  What  changes  have  been  made  in  the  general  construction  of  dynamos  dur- 
ing the  past  twenty  years? 

4.  Where  was  the  first  large  station  of  the  world  located?     When  was  it  started? 

5.  How  do  the  "Jumbo  "  dynamos  compare  with  those  of  to-day? 


FIG.  231.  —  Electric  Automobile. 


334  ELECTRICITY  AND    MAGNETISM 

6.  Describe  a  central  station  suitable  for  furnishing  light  and  power  to  a  small  city. 

7.  What  instruments  are  used  on  a  station  switchboard?     What  are  bus  bars? 

8.  How  are  shunt  dynamos  thrown  on  to  the  bus  bars,  or  into  parallel? 

9.  What  happens  if  a  dynamo  is  thrown  into  parallel  with  another  before  it  has 
developed  full  pressure? 

10.  \Vhat  happens  if  alternators  are  thrown  into  parallel  without  synchronizing? 

11.  \Vhy  are  double  pole  switches  almost  always  used  on  high  pressure  circuits? 

12.  Why  is  it  desirable  to  operate  dynamos  in  parallel? 

13.  How  are  the  feeders  arranged  in  an  alternating  current  station  where  the 
generators  are  not  run  in  parallel? 

14.  What  is  the  process  of  cutting  a  dynamo  out  of  circuit?     Why  is  the  load 
first  removed  ? 

15.  When  was  the  Richmond  electric  railroad  constructed? 

16.  Give  a  brief  history  of  the  development  of  the  electric  railroad. 

17.  What  is  a  trolley  wire?     A  trolley? 

1 8.  Describe  the  complete  electric  circuit  in  a  street  railway. 

19.  What  special  features  must  be  included  in  the  design  of  street  railway  motors? 

20.  WThy  are  two  motors  used  on  a  four-wheeled  car? 

21.  Why  is  approximately  500  volts  pressure   found  most   suitable   for   electric 
railways  ? 

22.  How  are  very  long  electric  railways  supplied  with  power  without  undue  loss 
in  the  wires? 

23.  What  is  a  track  bond?     Why  are  electric  railway  rails  bonded? 

24.  To  what  purposes  may  heavy  electric  locomotives  be  put? 

25.  How  are  output  records  kept  in  large  electric  stations? 

26.  Why  are  recording  voltmeters  especially  useful  in  a  lighting  station? 

27.  In  what  special  respect  does  the  load  of  an  ordinary  electric  railway  power 
plant  differ  from  that  of  an  electric  light  plant? 

28.  How  may  a  storage  battery  be  used  to  smooth  the  load  curve  of  an  electric 
station? 

29.  What  methods  are  used  for  controlling  street  car  motors? 

30.  Why  is  the  series-parallel  method  of  control  of  advantage  over  others? 

31.  On  what  does  the  torque  of  a  series  motor  depend? 

32.  On  what  does  the  speed  of  a  series  motor  depend  ? 

33.  Describe  a  motor  "starting  box." 

34.  Describe  the  operations  of  starting  and  stopping  a  motor. 

35.  For  what  purposes  may  electric  motors  be  advantageously  used? 

36.  WThy  do  electric  motors  form  ideal  devices  for  driving  the  machinery  of  manu- 
factories? 

37.  How  should  the  motors  be  arranged  in  an  electrically, driven  machine  shop? 

38.  How  is  power  supplied  to  an  electric  launch? 

39.  Describe  an  electric  automobile. 


CHAPTER   XIX 

THE  TELEGRAPH;    THE  TELEPHONE;    ELECTRIC   BELLS 

290.  Historical.  —  The  electric  telegraph,  as  we  know  it,  is  to  a  large 
degree  a  growth  from  the  discoveries  of  Professor  Joseph  Henry,  which 
were  directly  applied  to  the  purposes  of  telegraphy  by  S.  F.  B.  Morse 
and  his  assistants  ;  and  to  Morse  must  be  rightfully  ascribed  the  credit 
for  the  finished  telegraph  of  to-day.  His  name  is  written  irrevocably  in 
the  tale  of  the  world's  progress,  and  wherever  telegraphs  are  found  the 
name  of  Morse  accompanies  them. 

Numerous  attempts  to  produce  an  electric  telegraph  failed  during  the 
latter  part  of  the  eighteenth  century  and  the  first  three  decades  of  the 
nineteenth.  The  earlier  ones  were  dependent  upon  the  use  of  frictional 
machines  (which  proved  to  be  unreliable)  as  the  sources  of  electricity, 
for  Volta's  great  discoveries,  which  resulted  in  the  electric  battery,  were 
not  made  until  the  closing  years  of  the  eighteenth  century  and  the 
opening  years  of  the  nineteenth. 

Volta's  discoveries  were  followed  by  that  of  Oersted,  wherein  he  showed 
the  magnetic  effect  of  the  electric  current1;  and  close  on  the  heels  of 
these  followed,  in  1820,  Ampere's  demonstration  that  electric  currents 
exert  magnetic  forces  upon  each  other 2 ;  Arago's  and  Davy's  observa- 
tions that  steel  needles  may  be  magnetized  by  the  magnetic  effects  of 
electric  currents 3 ;  and  Schweiger's  discovery  of  the  increased  magnetic 
effect  caused  by  multiplying  the  electric  turns.4 

In  1825  the  essential  property  of  the  electromagnet  was  discovered 
by  Sturgeon,5  and  about  1830  Henry  made  the  magnet  as  it  stands 
to-day.  Henry  also  laid  civilization  under  unextinguishable  debt  by 
showing  the  way  to  the  most  powerful  use  of  electric  batteries  and  the 
working  of  electromagnets  at  great  distances.  These  discoveries  he  put 

i  Articles  119  and  120.  2  Articles  124  and  143.  3  Articles  125  and  126. 

4  Article  124.  5  Article  126. 

335 


336  ELECTRICITY  AND   MAGNETISM 

into  actual  use  in  the  construction  of  and  distant  working  of  relays  and 
electric  bells,  and  otherwise  extended  the  knowledge  of  the  laws  of 
electricity  and  magnetism. 

291.  Morse's  Invention.  —  The  time  was  ripe  for  a  practical  electro- 
magnetic telegraph,  and  early  in  1838  Morse  had  reached  a  satisfactory 
conclusion  of  his  series  of  inventions  begun  in  1832.  His  earlier  devices 
were  clumsy  and  complicated,  but  in  the  apparatus  of  1838  the  princi- 
ples embodied  in  the  earlier  attempts  were  accompanied  by  proper 
mechanical  designs,  and  worked  well. 

In  these  devices  the  magnet  was  not  unlike  that  of  the  present  day, 
but  all  signals  were  designed  to  be  recorded  on  slips  of  moving  paper. 
Much  battery  power  was  required  for  use  with  the  devices,  and  it  was 
impossible  to  transmit  the  necessary  operating  current  long  distances, 
and  Morse,  therefore,  introduced  Relays  into  his  circuits.  That  is, 
instead  of  causing  his  recorder  magnets  to  be  directly  worked  from  the 
line,  he  introduced  a  delicately  constructed  relay  magnet  which  was 
worked  by  the  line  current,  and  this  relay  in  turn  controlled  a  local  cir- 
cuit containing  the  recorder  magnet  and  a  Local  Battery. 

Since  Morse's  day  the  recorder  has  been  displaced  by  a  Sounder,  and 
other  simplifications  have  been  made,  but  the  essence  of  the  apparatus 
has  not  been  changed. 

292. .  First  Telegraph  Line.  —  The  first  telegraph  line  built  by  Morse 
was  constructed  by  means  of  an  appropriation  passed  by  Congress  in 
1845,  and  the  line  was  erected  to  connect  the  cities  of  Baltimore  and 
Washington. 

First  attempting  to  lay  the  wires  in  the  ground,  encased  in  lead  pipes, 
the  constructors  found  themselves  in  difficulty  from  imperfect  insulation, 
and  they  resorted  to  the  plan  of  stretching  cotton-covered  wires  over- 
head, on  poles  erected  for  the  purpose.  These  wires  were  supported 
at  the  poles  on  insulators  made  from  two  glass  plates  which  were  later 
replaced  by  glass  bureau  knobs.  The  construction  was  crude,  but. 
essentially  the  same  as  the  telegraph  and  telephone  construction  of 
to-day. 

The  line  was  opened  on  May  24,  1844,  with  the  much-quoted  mes- 
sage, "What  hath  God  wrought  ?,"  and  after  nearly  a  year  of  experi- 
mental demonstrations  it  was  opened  to  public  traffic.  It  soon  became 


THE  TELEGRAPH  337 

the  centre  of  interest  throughout  the  country,  and  the  construction  of 
telegraph  lines  was  quickly  undertaken  in  various  parts  of  the  land. 

293.  Needle  Telegraph.  —  Almost  coincident  with    the  development 
of  the  Morse  telegraph  in  America,  the  needle  telegraph  was  put  upon  a 
practical  footing  in  England.     The  operation  of  this  apparatus  depends 
upon  the  deflection  of  a  magnetic  needle  by  an  electric  current  in  a  sur- 
rounding coil.     By  making  and  breaking  the  circuit,  the  needle  may  be 
caused  to  swing  and  then  to  come  to  rest,  or  by  reversing  the  current 
the  needle  may  be  caused  to  swing  from  one  side  to  the  other ;  and  sig- 
nals may  be  indicated  by  the  motions  of  the  needle.     The  needle  tele- 
graph came  into  considerable  use,  but  it  has  now  been  almost  completely 
superseded  by  the  Morse  telegraph  and  the  code  of  signals  invented  by 
Morse,  which  consists  of  conveniently  spaced  dots  and  dashes,  and  is 
called  the  Morse  alphabet. 

It  may  be  properly  added  here  that  the  recognition  of  the  correctness 
of  the  laws  of  current  flow  enunciated  in  1827  by  Ohm,  though  tardy,  was 
still  of  great  service  in  the  later  development  of  telegraphy. 

294.  The  Telegraph  Key.  —  The  Morse  system  of  telegraphy  at  the 
present  time  has  four  elements  connected  in  series  : 

ist,  a  Battery  or  other  source  of  an  electric  current ; 

2d,  a  Key,  by  means  of  which  the  electric  circuit  may  be  made  and 
broken  to  produce  Signals  ; 

3d,  a  Line  of  wire  running  from  the  point  at  which  the  signals  are 
produced  to  the  point  where  the 
signals  are  to  be  received ; 

4th.  an  electromagnetic  Relay. 

BUT 

Sounder,  or  Register,  by  means 
of  which  the  signals  may  be  dis- 
tinguished or  recorded. 

The  battery  ordinarily  used  in 
telegraphy  is  the  common  grav- 
ity form  described  in  Article  49,  FIG.  232.  -  Ordinary  Form  of  TelegraphKey. 

but  galvanic  batteries,  during  the 

past  few  years,  have  been  widely  replaced  in  telegraph  service  by  small 

dynamos. 

The  ordinary  form  of  telegraph  key  is  shown  in  Figure  232.    This  con- 


338 


ELECTRICITY   AND   MAGNETISM 


FIG.    233.  —  Proper   Position   of   Operator's 
Hand  in  holding  Telegraph  Key. 


sists  of  a  lever,  which  is  pivoted  so  that  it  may  be  moved  through  a 
small  vertical  range  by  pressing  one's  fingers  upon  the  button  at  its 
end  (the  left  hand  of  the  figure).  The  spring  shown  at  the  centre  of 
the  figure  tends  to  keep  the  lever  at  the  upper  end  of  its  stroke,  so  that 

an  Operator,  in  making  signals 
with  the  key,  needs  only  depress 
the  lever  'and  it  will  return  to 
its  normal  position  upon  remov- 
ing the  pressure.  The  operator's 
fingers  are  therefore  placed  on 
the  button  in  the  way  shown  in 
Figure  233.  The  left-hand  Leg 
of  the  key  is  connected  to  a  contact  point  which  is  seen  directly  above 
the  leg  (Fig.  232),  and  which  is  insulated  from  the  frame.  The  lever 
carries  a  corresponding  contact  point  directly  above  the  insulated  one. 
The  upper  contact  point  is  in  electrical  contact  with  the  right-hand  leg 
through  the  metal  of  the  lever  and  frame.  When  the  key  is  connected 
into  a  circuit  by  cutting  the  circuit  wire  and  attaching  the  two  ends  to 
the  two  legs  of  the  key,  the  circuit  may  be  made  and  broken  at  the  will 
of  the  operator  by  depressing  or  raising  the  lever.  Some  keys  are  made 
without  legs,  but  with  binding  posts  for  the  connection  of  the  wires. 

As  ordinarily  arranged,  a  telegraph  circuit  is  broken  only  at  the  time 
of  making  signals ;  consequently  a  switch  is  placed  on  the  key  so  that 
the  circuit  can  be  closed 
when  the  key  is  not  in 
use.    The  handle  of  the 
switch  is  shown  in  Fig- 
ure 232,  just  to  the  left 
of  the   contacts.      With 
this  arrangement  of  the 
circuit  it  is  possible   to 
place  a  number  of  Sta- 
tions in  series  on  one  line  (Fig.  234)  ;  and  since  the  circuit  is  normally, 
complete,  —  that  is,  it  is  always  complete  when  not  in  use,  —  the  opera- 
tor at  any  station  may  signal  any  other  at  any  time,  provided  no  other 
operator  is  using  the  line. 


FlG.  234.  —  Diagrammatic  Illustration  of  Telegraph  Line 
with  Three  Stations. 


THE  TELEGRAPH 


339 


295.  The  Telegraph  Line.  —  The  current  used  in  telegraphy  is  quite 
small  —  it  does  not  often  exceed  fifty  milliamperes,1 —  consequently  it  is 
possible  to  satisfactorily  use  the  earth  for  one  side  of  the  circuit.     A  tele- 
graph line,  therefore,  ordinarily  consists  of  a  wire  supported  on  wooden 
poles  and  running  from  station  to  station.     At  its  ends  the  wire  is  con- 
nected to  the  earth  by  means  of  Ground  Plates,  as  shown  at  G,  G,  in 
Figure  234. 

The  wire  used  is  generally  made  of  the  best  galvanized  iron,  but 
for  some  short  lines  steel  wire  is  used,  and  for  some  of  the  most  impor- 
tant lines  between  large  cities  copper  wire  is  used.  Wires  as  large  as 
No.  4  Birmingham  wire  gauge  and  as  small  as  No.  10  are  sometimes 
used,  but  the  usual  size  is  No.  6  or  No.  8.  The  choice  of  the  size  and 
kind  of  wire  depends  largely  upon  the  length  and  importance  of  the  line, 
upon  which  also  depends  the  amount  of  battery  power  which  is  neces- 
sary to  operate  the  signals  and  the  care  which  is  lent  to  keeping  the 
line  in  good  condition. 

296.  Recording  Register.  —  In  the   earlier  days  of  the  Morse  tele- 
graph it  was  thought  necessary,  as  already  explained,   to  receive  the 
signals  constituting  a  telegraphic  message  in  a  permanent  form  by  means 
of  a  recording  register.    One  of  ^  ^P.APER_ROLL 
the  improved  registers  is  shown 

in  Figure  235.  This  consists 
of  a  case  containing  a  horse- 
shoe electromagnet,  the  wind- 
ings of  which  are  connected  in 
series  with  the  telegraph  cir- 
cuit. Over  the  poles  of  the 
magnet  is  an  Armature  of  soft 
iron,  which  is  held  against  a 
stop  by  the  pull  of  the  magnet 
when  the  current  flows  through 
the  circuit.  When  the  current 
is  interrupted  by  means  of  a 
key,  as  in  sending  signals,  the  electromagnet  loses  its  magnetism,  and 
the  armature  is  no  longer  attracted,  so  that  a  small  spring  which  is 

1  Article  194. 


FIG.  235.  —  Recording  Telegraph  Register. 


340  ELECTRICITY   AND    MAGNETISM 

attached  to  it  is  able  to  pull  it  back  from  the  stop.  Thus,  as  current 
impulses  are  sent  along  the  line  by  making  and  breaking  the  circuit  at  a 
key,  the  pulls  of  the  magnet  and  of  the  spring  alternately  draw  the 
armature  forward  and  backward. 

The  movement  of  the  armature  is  recorded  or  registered  by  means 
of  a  pen  or  a  blunt  point  on  a  strip  of  paper  which  is  automatically  fed 
from  the  roll  shown  in  the  figure  by  a  clockwork  mechanism  that  is 
inside  of  the  box.  This  paper  tracing  of  the  signals  may  be  read  by  the 
receiving  operator  and  translated  into  ordinary  language  upon  a  tele- 
graph-blank, and  the  latter  is  delivered  to  the  person  for  whom  the 
message  is  intended. 

297.  Telegraphic  Signals. — Telegraphic  signals  are  made  up  of  a 
combination  of  long  and  short  current  impulses,  which  are  made  by 
pressing  the  sending  key  at  proper  intervals  and  for  proper  periods,  and 
which  are  recorded  on  a  register  as  long  and  short  dashes.  Each  com- 
bination of  dashes  represents  a  letter  of  the  alphabet  or  a  certain  much- 
used  word  or  phrase.  The  Morse  Alphabet,  as  it  is  called,  which  is  used 
in  this  country,  is  given  below  (Fig.  236). 

MORSE  ALPHABET. 

A  B  C  _  ^  E  J^  G_ 

H  I  J  K.  L  M  N 


OP  Q  R  S  T  U 

v  w  x  Y  z  & 

NUMERALS 
12345 

678  90 


PUNCTUATION   MARKS 

PERIOD  COMMA  SEMI-COLON  COLON 


QUOTATION  MARK  PARENTHESIS  INTERROGATION 

PARAGRAPH  EXCLAMATION  DOLLAR  MARK 

FIG.  236. —  Morse  Alphabet  as  used  in  America. 

298.    Telegraph   Sounders.  —  As  the  Morse  telegraph  came  into  con- 
siderable use,  the  operators  found  that  they  could  read  the  signals  pass- 


THE  TELEGRAPH 


341 


ADJUSTING  SCREWS- 


ELECTRO  MAGNET 
SPRING 

DING  POSTS 


ing  over  the  line  by  listening  to  the  clicks  of  the  register  armature  as  it 
moved  back  and  forth  between  its  stops  under  the  influence  of  the  cur- 
rent impulses.  The  paper  roll  was,  therefore,  abandoned,  as  "  reading 
by  sound "  was  quicker  and  more  convenient  than  translating  the 
message  from  the  paper  tracing  of  the  signals. 

To  make  reading  by  sound  as  easy  as  possible,  the  working  mechanism 
of  the  register  was  altered  into  that  of  the  Sounder,  and  Figure  237 
plainly  shows  its  arrangement. 
The  armature,  which  is  of  soft 
iron,  has  attached  to  it  a  sub- 
stantial brass  bar.  This  bar  is 
pivoted  at  its  right-hand  end, 
as  shown  in  the  figure,  so  that 
its  left-hand  end  may  move  up 
and  down  between  adjustable 
stops,  as  shown.  To  the  right- 
hand  end  of  the  bar  is  attached 
a  spring  which  draws  the  bar 
against  the  upper  stop  when 
no  current  is  flowing  in  the  magnet.  The  large  cylinders  shown  about 
the  centre  of  the  figure  compose  the  magnet.  This  magnet  consists  of 
two  cores  of  iron  about  three-eighths  of  an  inch  in  diameter  and  one 
inch  and  a  half  long,  wound  with  wire  and  covered  with  black  paper  or 
a  short  piece  of  hard  rubber  tube.  The  cores  are  screwed  fast  to  an 
iron  base  so  as  to  make  a  horseshoe  electromagnet.  The  armature  is 
shown  at  the  top  of  the  figure,  above  the  magnet. 

When  current  flows  in  the  magnet  winding,  the  armature  is  attracted 
and  the  bar  drawn  against  the  lower  stop.  As  the  bar  moves  back  and 
forth  it  makes  a  sharp  click  whenever  it  strikes  one  of  the  stops.  The 
strength  of  the  spring  is  adjustable  by  means  of  a  screw,  so  that  the 
sounder  may  be  adjusted  for  use  within  a  certain  range  of  currents  of 
different  strengths. 

To  successfully  read  signals  from  a  sounder,  much  experience  is 
necessary,  but  operators  become  very  expert  by  long  practice.  It  is 
necessary  in  reading  to  distinguish  between  the  clicks  of  the  armature 
against  the  top  and  bottom  stops.  A  little  consideration  will  show  that 


FIG.  237.  —  Telegraph  Sounder. 


342 


ELECTRICITY   AND   MAGNETISM 


ARMATURE  CONTACT 
POINTS 


ELECTRO  MAGNET 


the  length  of  time  between  the  clicks  when  the  armature  strikes  the 
bottom  stop  and  when  it  strikes  the  top  stop  distinguishes  between  dots 
and  dashes,  since  the  dots  and  dashes  represent  intervals  during  which 
current  is  flowing  through  the  magnet.  The  interval  of  time  between 
the  top  click  and  the  bottom  click  represents  the  spacing  between  the 
dots  and  dashes,  because  the  spacing  represents  .intervals  during  which 
no  current  flows,  or  during  which  the  signal  key  is  open. 

299.  Relays.  —  Telegraph  sounders  require  only  a  fraction  of  an  am- 
pere to  operate  them,  but  to  cause  that  fraction  to  flow  through  a  long 

line,  which  necessarily  has  a  high 
resistance,  requires  the  use  of  a 
battery  of  a  very  large  number 
of  cells.  This  is  undesirable, 
because  the  cells  are  expen- 
sive to  buy  and  to  keep  up.1 
Long  telegraph  lines  are,  there- 
fore, furnished  with  instruments 
which  operate  like  sounders,  but 
which  are  made  very  sensitive 
by  placing  a  great  many  turns 

of  fine  wire  on  their  magnets,  so  that  they  may  be  satisfactorily  operated 
on  as  little  current  as  eight  or  ten  milliamperes.  These  instruments  are 
called  Relays  (Fig.  238). 

Reading  signals  directly  from  a  relay  is  not  usually  attempted,  as  the 
motion  of  its  armature  is  so  delicate  that  it  makes  very  little  sound,  but 
the  armature  and  one  of  its  stops  are  arranged  as  a  part  of  a  Local 
Circuit,  which  contains  a  sounder  and  a  couple  of  gravity  cells  (Fig. 
239).  As  the  relay  armature  moves  back  and  forth  it  makes  and  breaks 
the  local  circuit  and  reproduces  in  it  the  signals  which  pass  over  the 
main  line.  The  sounder  in  the  local  circuit  gives  the  signals  exactly  as 
they  pass  over  the  line. 

300.  Multiple  Telegraphy. — To  still  further  economize  in  long  and 
important  lines,  arrangements  are  made  to  send  more  than  one  message 
at  a  time  over  each  wire.     When  a  telegraph  wire  is  arranged  so  that 
two  messages  may  be  transmitted  over  the  wire  at  once,  one  being  sent 

1  Article  54. 


FIG.  238.  — Telegraph  Relay. 


THE  TELEGRAPH 


343 


from  each  end,  the  wire  is  said  to  be  Duplexed.  When  it  is  so  arranged 
that  both  messages  may  be  sent  from  one  end,  the  wire  is  usually  said 
to  be  Diplexed.  Di- 
plexed  wires  are  not 
ordinarily  used,  ex- 
cept in  combination. 
When  a  wire  is  ar- 
ranged so  that  four 
messages  may  be 
transmitted  over  it 
at  once,  two  being 
sent  from  each  end, 
it  is  said  to  be  Quad- 
ruplexed.  In  arrang- 
ing a  Quadruplex,  a 
combination  is  prac- 
tically made  of  a 
duplex  and  a  diplex 
arrangement. 

301.  Duplex  Telegraphy. — The  commonest  arrangement  for  duplex 
telegraphy  requires  a  special  relay,  which  is  connected  as  shown  in 
Figure  240.  It  is  seen  that  three  points  of  connection  are  made  to  the 
wire  which  is  wound  on  the  electromagnet  of  the 
relay,  one  point  at  each  end  of  the  wire,  and  a  third 
at  the  middle  of  the  wire.  One-half  of  the  wire  is 


Key 


Local  Battery 

FIG.  239.  —  Illustration  of  Apparatus  at  Telegraph  Station, 
including  Key,  Relay,  and  Local  Circuit,  with  Battery 
and  Sounder. 


1 


wound  on  each  leg  of  the  electromagnet  in  the  usual 
way.  If  a  current  is  passed  through  the  wire  from 
one  end  to  the  other,  as  indicated  by  the  arrow-heads 
on  the  wire,  the  relay  acts  as  a  common  relay. 

If  a  current  is  sent  into  the  relay  from  the  middle 
point,  the  current  divides,  as  indicated  by  the  dotted 
arrows,  and  the  two  parts  pass  through  the  windings 
on  the  two  legs  of  the  electromagnet  in  such  a  way 
that  their  magnetic  effects  are  in  opposite  directions. 
If  the  two  divisions  of  the  current  are  equal,  their  magnetic  effects 
neutralize  each  other,  so  that  the  armature  of  the  relay  is  not  affected. 


MIDDLE 

FIG.  240.  —  Diagram 
of  Differential  Re- 
lay for  Duplex  Te- 
legraphy. 


344 


ELECTRICITY  AND   MAGNETISM 


Such  a  relay  is  called  a  Differential  Relay.     The  exact  arrangement 
of  the  windings  on  a  differential  relay  may  vary  considerably,  but  the 
purpose  and  effect  of  all  arrangements  are  exactly  the  same  as  described. 
LINE  WIRE  Figure    241    is   a  diagram 

of  the  connections  made  at 
a  telegraph  station  for  duplex 
telegraphy  using  a  differen- 


GROUND PLATE 


FIG.  241. —  Diagram  of  Instruments  and  Circuits 
at  a  Telegraph  Station,  arranged  to  operate 
by  the  Differential  Duplex  System.  N,  Differ- 
ential Relay ;  r±  and  r2>  Special  Resistances. 


tial  relay.  This  arrangement 
is  often  called  the  Differential 
Duplex.  The  figure  shows  by 
the  arrowheads  that  the  cur- 
rent sent  into  the  line  at  the 
station  divides  in  the  relay 
belonging  to  that  station,  and 
half  of  it  passes  to  the  ground 
through  the  resistance  (fi)  and  back  to  the  battery.  The  other  half  of 
the  current  goes  through  the  line  and  the  relay  of  the  distant  station  for 
which  the  signals  are  intended,  and  returns  to  the  battery  by  way  of  the 
earth. 

The  two  halves  of  the  current  pass  through  the  coils  of  the  home 
relay  in  opposite  directions  and  neutralize  each  other's  magnetic  effects. 
A  message  sent  from  the  home  key,  therefore,  does  not  affect  the  home 
relay.  That  part  of  the  current  which  goes  into  the  line  passes  through 
the  winding  of  the  distant  relay  in  the  usual  manner,  so  as  to  make  a 
signal.  In  this  way  the  operator  at  any  station  on  a  line  can  signal 
another  without  affecting  his  home  relay.  Two  messages  can  therefore 
be  transmitted  at  the  same  time  in  opposite  directions  between  two 
stations  without  interference.  This,  of  course,  requires  two  operators, 
one  to  send  and  one  to  receive  the  message  at  each  station.  The  figure 
shows  a  special  device  for  making  the  signals,  which  is  called  a  Trans- 
mitter. This  is  explained  in  the  next  article. 

In  order  that  a  differential  duplex  system  may  work,  it  is  absolutely 
necessary  that  the  current  in  the  relay  divide  quite  accurately  into  halves. 
This  is  effected  by  properly  adjusting  the  resistance  of  the  rheostat  R  in 
the  home  branch  of  the  circuit.  The  branch  of  the  circuit  at  each  sta- 
tion containing  the  resistance  R  is  called  the  Artificial  Line,  since  it  is 


THE  TELEGRAPH  345 

made  to  represent  as  far  as  possible  the  condition  of  the  actual  line  in 
order  that  the  duplex  may  work  satisfactorily.  The  resistance  of  the  line 
is  easily  balanced  by  making  the  resistance  R  so  that  it  may  be  adjusted 
by  plugs  to  suit  the  condition  of  the  line.  Certain  smaller  resistances  (^ 
and  r*  in  the  figure)  are  also  used  in  the  home  circuits  to  smooth  the 
action  of  the  transmitter  in  making  and  breaking  the  circuit. 

The  electrostatic  capacity  of  the  line  affects  the  rise  and  fall  of  the 
current  in  it  as  the  signals  are  transmitted,  and  in  order  to  get  the  best 
results  with  the  differential  relay  the  artificial  line  is  arranged  with  a 
condenser,  S,  connected  in  parallel  with  the  resistance  R,  the  capacity 
of  which  balances  that  of  the  line.  This  condenser  is  usually  made  of 
sheets  of  tin-foil  insulated  by  sheets  of  thin  mica  or  paraffined  linen 
paper,  and  the  capacity  connected  into  circuit  may  be  adjusted  by  means 
of  plugs. 

When  a  telegraph  circuit  is  arranged  for  duplex  working,  it  is  necessary 
to  have  a  line  battery  at  each  of  the  stations.  For  simple  working  all 
the  battery  may  be  placed,  if  desired,  at  a  single  point  along  the  line. 

302.  Diplex  Working.  —  When  a  line  is  arranged  for  diplex  working, 
two  keys  are  placed  at  the  sending  station  and  two  relays  are  placed  at 
the  receiving  station.  One  of  these  relays,  called  the  Polarized  Relay, 
has  a  permanently  magnetized  steel  armature.  When  the  armature, 
which  lies  across  the  poles  of  a  horseshoe  electromagnet  is  permanently 
magnetized  or  polarized,  it  is  attracted  when  the  current  flows  in  one 
direction  and  repelled  when  the  current  flows  in  the  opposite  direction. 

Advantage  may  best  be  taken  of 
this  by  placing  the  polarized  arma- 
ture between  the  poles  of  the  elec- 
tromagnet, as  in  Figures  242  and 

24  T.     The  end  of  the  armature  will 
FIG.  242.  — Diagram  .  .  .  FIG.  243.  —  Diagram 

of  Polarized  Relay    then    Stick    against    Cither    pole    111-        Of  Polarized  Relay 

with  Current  flow-  differently  when  no  current  flows,  with  Current  flow- 
ing from  Left  to  .-  .  ,  ,  .  ing  from  Right  to 
Ri*ht  if  it  is  not  restrained  by  a  spring.  L*ft 

When  a  current  flows  in  one  direc- 
tion, the  end  of  the  armature  will  move  up  to  one  pole,  and  when  the 
current  is  reversed,  the  armature  will  move  over  to  the  other  pole,  as 
shown  in  the  figures. 


34^ 


ELECTRICITY   AND   MAGNETISM 


ELECTRO    MAGNET 


CONTACT  POINTS 


A  polarized  relay  made  upon  this  principle  may  be  operated  by  sig- 
nals which  are  given  by  reversing  the  current  in  the  circuit  instead  of 

making  and  breaking  the  circuit  as 
in  simple  telegraphy. 

Figure  244  shows  a  common  form 
of  polarized  relay  in  which  the  ar- 
mature is  kept  polarized  by  means 
of  a  strong  permanent  magnet  to 
which  it  is  attached.  A  key  for 
sending  signals  by  reversing  the 
current  is  called  a  Pole  Changer. 

It  is  possible  to  send  signals  to 
a  common  or  neutral  relay  over 
the  same  line  as  that  used  with  the 
polarized  relay,  without  interfering 
with  the  action  of  the  latter.  In 
order  that  the  currents  which  ac- 

FlG.  244.  —  Commercial  Form  of  Polarized 

Telegraph  Relay.  tuate  the  polarized  relay  shall  not 

also    work    the    neutral    relay,    the 

latter  is  adjusted  to  respond  only  to  a  current  which  is  greater  than  that 
required  to  actuate  the    polarized  relay,  and  we  thus  have  a  diplex 


SENDING  STATION 


RECEIVING  STATION 


FIG.  243.  — Diagram  of  Instruments  and  Circuits  at  Sending  and  Receiving  Telegraph 
Stations  arranged  to  operate  by  the  Two-current  Diplex  System.  #](  Small  Battery  ;  />'.,, 
Large  Battery;  A",,  Pole  Changer  and  Key;  K±t  Transmitter  and  Key;  /\,  Polarized 
Relay ;  P2,  Neutral  Relay. 

arrangement.     Hence  the  operation  of  the  diplex  arrangement  depends 
upon  the  use  of  currents  of  two  strengths.     One  of  these  currents  is 


THE  TELEGRAPH 


347 


quite  weak  and  is  reversed  in  sending  signals,  so  that  a  polarized  relay 
is  used  with  it ;  the  other  current  is  stronger  and  is  increased  and 
decreased  by  the  sending  key  in  sending  signals  to  the  neutral  relay, 
instead  of  making  and  breaking  the  circuit,  since  doing  the  latter  would 
interfere  with  the  signals  sent  to  the  polarized  relay. 

A  key  arranged  to  increase  and  decrease  the  current  in  sending  sig- 
nals is  usually  called  a  Transmitter.  The  increase  and  decrease  of  the 
current  is  gained  by  alternately  connecting  into  circuit  a  large  and  a 
small  battery  (Fig.  245).  In  order  that  common  telegraph  keys  may 
be  used  by  the  operators  in  sending  messages  by  the  diplex  arrangement, 
it  is  usual  to  work  the  pole  changer  and  transmitter  by  means  of  electro- 
magnets like  sounders,  connected  individually  in  local  circuits  in  series 
with  the  sending  keys.  Figure  245  is  a 
diagram  of  the  connections  at  the  sending 
and  receiving  stations  upon  a  diplex  tele- 
graph line. 

303.  Quadruplex  Telegraphy.  —  The  com- 
monly used  quadruplex  system  is  essentially 
a  combination  of  the  differential  duplex  and 
two-current  diplex  which  have  just  been  ex- 
plained. A  diagram  of  the  circuits  at  a 
quadruplex  station  is  shown  in  Figure  246. 

It  is  to  be  seen  in  the  diagram  that 
polarized  and  neutral  relays  of  the  diplex 
arrangement  are  used,  but  they  are  wound 
in  differential  fashion.  This  makes  it  pos- 
sible to  send  two  messages  from  a  station 
without  interference  with  each  other  (di- 
plex), and  also  without  interference  with 
receiving  two  messages  at  the  same  station 
by  means  of  the  differential  instruments. 

The  key  arrangements  for  the  quadru- 
plex system  are  the  same  as  those  of  the 
diplex  system,  so  that  a  pole  changer  and 
transmitter  are  used,  though  for  simplicity 
they  are  not  fully  shown  in  Figure  246. 


GROUND  PLATE 

FIG.  246.  —  Diagram  of  Instru- 
ments and  Circuits  at  Tele- 
graph Station  arranged  to 
operate  by  the  Quadruplex 
System.  G±,  G.2,  Dynamos; 
K^  Pole  Changer ;  A"2>  Trans- 
mitter and  Key  ;  P±,  Polarized 
Differential  Relay;  />2,  Neu- 
tral Differential  Relay;  Ki,R2, 
Adjustable  Resistances ;  S, 
Condenser. 


348 


ELECTRICITY   AND   MAGNETISM 


The  figure  shows  the  use  of  dynamos  in  the  place  of  batteries.  The  coils 
marked  700  and  1000  are  resistances  which  take  the  place  of  the  resist- 
ances ri  and  r.>  in  Figure  241.  The  coil  marked  1400  is  used  to  reduce 
the  current  which  flows  through  the  neutral  relay  between  the  signals, 
and  it  fulfils  the  purpose  of  the  division  of  the  battery  into  a  large  and  a 
small  section.1  The  numbers  represent  the  resistances  of  the  coils  in 
ohms.  The  signals  for  the  neutral  relay  are  made  by  alternately  cutting 
this  large  resistance  into  and  out  of  the  circuit  by  the  transmitter.  The 
action  can  be  understood  by  an  examination  of  the  illustration. 

For  satisfactory  quadruplex  working  the  artificial  line  must  be  kept 
well  adjusted,  or  trouble  is  experienced  from  blurring  the  signals  in  the 
differential  instruments. 

304.  Miscellaneous  Telegraph  Systems.  —  Another  duplex  arrange- 
ment, which  depends  for  its  operation  on  a  balance  similar-  to  that  of  a 
Wheatstone  bridge,  is  often  used  on  ocean  cables.  The  arrangement  is 
shown  in  Figure  247.  In  this  system  the  relay  is  located  at  Z.  In  the 
arms  AC  and  AB  of  the  triangle,  fixed  resistances  are  located  ;  R  is  a 

variable  resistance,  and  S  is  a.  condenser 

t  INF 

which  shunts  R.  Now,  according  to  the 
principle  of  the  Wheatstone  bridge,  if  the 
resistance  in  ^Cis  to  the  resistance  in 
AB  as  the  resistance  of  the  line  is  to  the 
resistance  R,  no  current  will  flow  fr.om 


GROUND  PLATE 


ment. 


GROUND  PLATE 


the  ho™  Battery  through  the  re.ay  /. 
Current  from  a  distant  battery  will,  how- 
ever, work  the  relay,  since  it  will  come 

in  from  the  line  to  C,  where  it  will  divide,  and  a  part  will  go  around 
through  CA  in  its  path  to  earth,  but  the  greater  part  will  pass  directly 
through  the  relay,  which  offers  a  path  of  lower  resistance.  The  con- 
denser S  is  used  to  balance  the  effect  of  the  capacity  of  the  line. 

A  duplex  system  may  also  be  operated  by  means  of  differential  polar- 
ized relays.  This  is  practically  the  same  in  operation  as  the  differential 
duplex  systems  already  explained,  but  pole  changers  are  used  to  send 
the  signals,  and  polarized  relays  are  used  to  receive  them. 

Various  plans  for  sending  more  than  four  messages  over  one  wire  have 

1  Article  302  and  Figure  245. 


THE   OCEAN  TELEGRAPH  349 

been  devised,  but  they  have  been  too  complicated  to  be  successful  in 
operation,  and  can  receive  no  attention  here.  When  more  than  four 
messages  are  sent  over  a  wire,  the  arrangement  is  ordinarily  called  Mul- 
tiplex Telegraphy,  though  this  title  really  applies  to  all  telegraphic  ar- 
rangements where  two  or  more  messages  are  sent  over  a  wire  at  one  time. 

Methods  have  also  been  devised  for  sending  messages  by  machines 
instead  of  by  hand.  Such  machines  are  used  to  a  considerable  extent 
for  special  work,  such  as  sending  press  despatches,  stock  quotations,  and 
similar  purposes.  It  is  usual  for  machine-sent  messages  to  be  received 
by  special  machine  recorders,  which  print  the  messages  either  in  Morse 
characters  or  directly  in  the  English  alphabet.  Where  machines  are 
used  in  telegraphy,  the  arrangement  is  ordinarily  spoken  of  as  Auto- 
matic Telegraphy. 

Other  devices  have  been  invented  by  means  of  which  a  written  mes- 
sage or  sketch  may  be  transmitted  exactly  as  it  is  written  or  drawn. 
These  are  ordinarily  spoken  of  as  devices  for  Autographic  Telegraphy. 
The  most  successful  of  these  arrangements  thus  far  is  the  Gray's  Telau- 
tograph, but  none  of  them  have  yet  come  into  general  use,  on  account 
of  their  complications. 

305.  Ocean  Telegraphy.  —  Before  passing  from  the  subject  of  teleg- 
raphy to  the  closely  allied  one  of  telephony,  it  is  worth  while  to  say  a 
word  concerning  Submarine  telegraphy.  The  first  successful  cable  was 
laid  from  America  to  Europe  in  1866.  The  result  is  largely  due  to  the 
tireless  energy  and  exceptional  grit  of  Cyrus  W.  Field,  an  American 
capitalist.  Mr.  Field  first  entertained  the  thought  of  carrying  "  the  line 
across  the  ocean"  in  the  early  part  of  1854.  He  at  once  formed  a 
company,  and,  though  met  by  ridicule  and  rebuffs  on  every  hand  (dealt 
equally  by  scientific  men  and  capitalists),  he  succeeded  in  enlisting  the 
aid  of  friends  on  both  sides  of  the  Atlantic  and  the  interest  of  the  gov- 
ernments of  the  United  States  and  Great  Britain.  After  several  attempts, 
and  the  partial  loss  of  two  cables,  a  cable  was  finally  laid  in  1858  across 
the  ocean  from  Ireland  to  Newfoundland.  It  transmitted  congratula- 
tions between  the  President  of  the  United  States  and  the  Queen  of 
England ;  worked  with  difficulty  three  weeks ;  and  then  became  silent. 

This  failure,  due  to  weakness  in  the  construction  of  the  cable,  was  a 
crushing  blow.  The  company  had  spent  millions  and  its  supporters 


35O  ELECTRICITY  AND    MAGNETISM 

were  losing  confidence,  but  Field  had  the  same  unconquerable  determi- 
nation as  Morse.  He  managed  to  maintain  the  enterprise, and  as  early 
as  1863  arrangements  were  put  on  foot  looking  to  the  laying  of  another 
cable.  This  delicate  work  was  begun  in  1865  from  the  noted  ship  Great 
Eastern.  In  the  first  trial  one  cable  was  entirely  lost  by  parting  at  sea 
when  nearly  laid,  but  after  several  further  unsuccessful  attempts  the  lay- 
ing of  a  cable  was  properly  accomplished  in  1866,  and  it  was  put  into 
successful  operation.  The  lost  cable  of  1865  was  soon  after  grappled 
for,  found,  brought  to  the  surface,  spliced,  and  completed ;  and  it,  too, 
was  put  into  operation.  These  two  cables,  and  numerous  later  ones, 
have  continued  in  satisfactory  service. 

306.  Working  of  Ocean  Cables.  —  The  cables  for  submarine  telegra- 
phy consist  of  one  or  two  copper  conductors  embedded  in  a  thick  insu- 
lation of  gutta-percha,  or  an  equally  good  insulator,  which  is  protected 
from  injury  in  handling  by  a  thick  covering  of  jute  fibre  and  one  or 
more  wrappings  of  heavy  iron  wire.  The  conductors  are  made  of 
several  strands  of  small  copper  wire  twisted  together  for  the  purpose  of 
making  the  cable  flexible. 

The  apparatus  which  is  now  used  for  receiving  the  signals  is  of  a  re- 
cording type  of  D'Arsonval  galvanometer  called  a  siphon  recorder.  It  is 
infinitely  more  delicate  and  sensitive  than  that  described  in  Article  296. 
The  first  successful  receiving  apparatus  was  designed  by  Lord  Kelvin. 
It  consisted  of  a  very  fine  galvanometer  of  high  resistance,  which  by 
the  swing  of  its  needle  moved  a  very  light  feather  quill  that  made  the 
record ;  or,  the  galvanometer  alone  was  used,  and  an  operator  read  the 
signals  directly  from  the  swing  of  the  needle.  The  Kelvin  galvanome- 
ters, developed  during  the  laying  of  the  early  cables,  still  remain  models 
of  inventive  skill  and  constructive  perfection. 

The  recording  devices  for  ocean  cables  may  either  be  placed  in  direct 
connection  between  the  end  of  the  cable  and  the  ground,  or  the  cable 
may  be  connected  to  one  set  of  the  plates  of  a  condenser.  The  other  set 
of  plates  is  then  connected  through  the  galvanometer  to  the  ground.  In 
this  case  the  charge  passing  into  the  condenser  from  the  ground,  under 
the  influence  of  the  charge  from  the  cable,  causes  the  galvanometer  de- 
flection. The  transmitter  consists  essentially  of  a  key  that  either  throws 
one  terminal  of  a  strong  battery  directly  upon  the  cable,  or  charges  the 


THE  OCEAN  TELEGRAPH  351 

plates  of  a  condenser  connected  as  explained  above.  One  terminal  of 
the  battery  must,  of  course,  be  connected  to  the  ground. 

307.  Duplex  Cable  Working.  —  It  is  usual  nowadays  to  work  cables 
duplexed  by  means  of  an  arrangement  of  circuits  called  the  "  bridge 
duplex  "  system,  which  is  described  in  Article  304  and  illustrated  in  Fig- 
ure 247.  The  receiving  instruments  at  each  station  are  represented  by 
the  letter  Z,  and  the  sending  key  and  battery  by  the  letters  K  and  b. 
The  devices  marked  R  and  S  are  resistances  and  condensers  for  the  pur- 
pose of  adjusting  the  operation  of  the  instruments,  and  fixed  equal 
resistances  are  placed  in  the  arms  AC  and  AB. 

When  either  key  is  depressed  so  as  to  put  current  upon  the  cable,  the 
home  receiving  instrument  is  not  affected,  provided  the  resistance  and 
capacity  of  the  artificial  cable,  R,  S,  are  equal  to  those  of  the  real  cable 
added  to  those  of  the  arrangements  of  the  far  station.  This  is  in  accord- 
ance with  the  law  of  the  Wheatstone  bridge  : 

Tr  AC         Real  Cable 

If  — -  =  ..,..„,.,  no  current  will  pass  through  Z. 
AB      Artificial  Cable 

Now,  when  a  current  comes  into  a  station  from  the  cable,  its  easiest 
path  to  the  ground,  by  which  it  may  return  to  its  battery,  is  through  the 
siphon  recorder  Z  ;  and  the  signal  record  is  a  result  of  this  current  flow. 

QUESTIONS 

1.  Who  invented  the  modern  telegraph?     When? 

2.  Give  a  brief  history  of  the  discoveries  in  electricity  which  made  possible  an 
electromagnetic  telegraph. 

3.  Who  was  Henry?     What  did   he  do  toward   laying  the  foundation  of  the 
telegraph? 

4.  When  was  the  first  Morse  telegraph  line  built?     Tell  about  it. 

5.  What  is  the  needle  telegraph ?     How  does  it  work? 

6.  When  did  Ohm  announce  the  law  that  bears  his  name? 

7.  What  are  the  essential  elements  of  an  electric  telegraph? 

8.  How  is  the  current  obtained  for  telegraph  lines? 

9.  Describe  a  telegraph  key. 

10.  How  are  telegraph  signals  made? 

11.  What  is  the  advantage  of  having  the  current  flow  through  the  line  at  all  times 
except  when  signals  are  being  sent? 

12.  Why  must  a  switch  be  used  with  the  key? 


352  ELECTRICITY   AND   MAGNETISM 

13.  What  kinds  and  sizes  of  wire  are  used  in  telegraph  lines? 

14.  How  does  a  recording  register  work? 

15.  How  are  letters  and  words  composed  of  telegraph  signals? 

1 6.  Describe  a  sounder. 

17.  How  are  signals  read  from  a  sounder? 

1 8.  Describe  a  relay. 

19.  How  are  relays  used?     What  is  the  local  circuit? 

20.  Why  are  relays  used? 

21.  What  is  multiple  telegraphy? 

22.  What  is  duplex  telegraphy? 

23.  What  is  diplex  telegraphy? 

24.  What  is  quadruplex  telegraphy? 

215.  What  is  a  differential  relay?     How  is  it  used? 

26.  What  is  the  principle  of  a  differential  duplex  system? 

27.  What  is  the  artificial  line? 

28.  What  is  a  pole  changer?     What  is  it  for? 

29.  \Vhat  is  the  principle  of  a  two-current  diplex  system? 

30.  What  are  the  principles  of  a  quadruplex  system? 

31.  How  must  the  relays  be  wound  for  differential  duplex  working? 

32.  Explain  the  principle  of  the  bridge  duplex. 

33.  Why  does  the  home  relay  in  the  bridge  duplex  not  record  the  signals  sent 
from  the  home  transmitter? 

34.  What  are  automatic  and  autographic  telegraphs? 

35.  When  was  the  first  Atlantic  cable  laid? 

36.  What  troubles  did  Mr.  Field  encounter  in  laying  a  cable? 

37.  How  are  ocean  cables  worked? 

308.  The  Telephone.  —  Unlike  telegraphy,  which  is  nearly  the  oldest 
commercial  application  of  electricity,  the  telephone  is  one  of  the  later 
commercial  applications.  The  word  "  telegraph  "  comes  from  two  Greek 
words  which  mean,  when  combined,  to  write  at  a  distance,  while  "  tele- 
phone "  comes  from  two  Greek  words  which  mean  to  speak  at  a  distance. 
The  first  telephone  that  can  be  given  the  credit  of  commercial  success 
was  invented  by  Alexander  Graham  Bell,  and  was  privately  exhibited  by 
him  at  the  Centennial  Exposition  at  Philadelphia  in  1876.  Dr.  Elisha 
Gray  applied  for  patents  on  a  telephone  mechanism  at  the  same  time, 
and  the  nearly  simultaneous  invention  of  the  instrument  by  the  two 
noted  men  gave  rise  to  a  famous  patent  law  suit. 

Since  that  time  the  usefulness  of  the  telephone  has  been  greatly 
increased  by  other  inventions  which  make  its  service  more  perfect.  In 


THE  TELEPHONE 


353 


its  improved  form,  it  has  added  wonderfully  to  the  ease  and  quickness 
with  which  many  kinds  of  business  may  be  transacted,  and  it  may  be 
said  to  have  revolutionized  many 
of  the  processes  of  doing  busi- 
ness. 

The  telephone  originally  ex- 

,.,.,,       -T.   ,,  •  A   j     r  ^  FIG.   248. —  Illustration   of  Bell's   Early  Tele- 

hibited  by  Bell  consisted  of  two  phone  System> 

instruments  quite  similar  to  the 

ear  pieces  or  Receivers  which  are  now  used.  One  of  these  instruments 
was  used  as  a  receiver  and  the  other  was  used  to  talk  into,  or  as  a 
Transmitter,  and  the  two  were  connected  by  wires  (Fig.  248). 

The  construction  of  these  instruments  may  be  best  explained  by  refer- 
ence to  Figure  249,  which  is  an  illustration  of  a  late  type  of  Bell  receiver. 
In  this  figure  R  represents  a  rubber  case,  NS  a  magnet  tipped  with  a  piece 
of  soft  iron,  and  /Fa  spool  of  very  fine  wire  slipped  over  the  soft  iron  tip 
of  the  magnet  and  connected  to  the  binding  posts,  PP,  at  the  end  of  the 
rubber  case.  D  is  a  Diaphragm  or  disk  made  of 
thin  varnished  iron.  This  diaphragm  is  firmly 
clamped  all  round  its  edges  in  such  a  position 
that  its  centre  is  very  close  to  the  end  of  the 
magnet  NS.  When  one  of  the  instruments  is 
brought  close  to  a  speaker's  mouth,  the  waves  of 
sound  caused  by  his  speech  strike  the  diaphragm 
and  cause  it  to  vibrate,  or  move  back  and  forth. 
The  speaking  end  of  the  instrument  is  formed  into 
a  Mouthpiece  of  such  a  shape  that  it  gathers  in  a 
large  volume  of  the  waves  of  sound,  and  concen- 
trates their  effect  upon  the  diaphragm. 

Most  of  the  magnetic  lines  of  force1  belonging 
to  the  magnet  pass  through  the  coil  of  wire,  W, 
and  some  of  them  enter  the  iron  diaphragm  on 
their  path  to  the  opposite  pole.  As  the  diaphragm  vibrates  from  the 
effect  of  a  voice,  it  moves  back  and  forth  in  front  of  the  magnet.  These 
vibrations  are  very,  very  small, — entirely  too  small  to  be  seen  by  the 
eye,  —  but  they  are  of  sufficient  extent  to  cause  the  number  of  lines  of 

1  Articles  84  and  85. 
2A 


FIG.  249.  —  Sectional 
View  of  Bell  Tele- 
phone Receiver. 


354  ELECTRICITY   AND   MAGNETISM 

force  which  enter  the  disk  to  increase  considerably  as  it  approaches  the 
magnet,  and  decrease  as  it  moves  away  from  the  magnet.  In  this  way 
the  distribution  of  the  lines  of  force  around  the  end  of  the  magnet  is 
altered  with  each  movement  of  the  disk,  and  the  number  of  lines  of 
force  which  pass  through  the  coil,  W,  of  wire  on  the  magnet  is  increased 
or  decreased  at  the  same  time. 

It  is  an  experimentally  determined  fact  that  when  a  change  occurs  in 
the  number  of  lines  of  force  passing  through  a  coil,  an  electric  pressure 
is  set  up  in  the  coil.1  This  pressure  is  in  one  direction  when  the  num- 
ber of  lines  of  force  passing  through  the  coil  is  increased,  and  in  the 
opposite  direction  when  the  number  is  decreased.  Consequently  the 
movements  of  the  Bell  telephone  diaphragm  set  up  electric  pressures  in 
the  telephone  coil,  and  when  this  coil  is  connected  by  wire  to  the  coil 
of  another  telephone,  as  in  Figure  248,  waves  of  current  flow  through  the 
circuit  which  correspond  in  a  general  way  to  the  waves  of  sound  set  up 
in  front  of  the  diaphragm  of  the  first  telephone.  As  these  current  waves 
flow  through  the  coil  of  the  second  telephone,  they  increase  and  decrease 
the  strength  of  its  magnet.  This  alters  the  amount  of  the  attraction 
which  the  magnet  exerts  on  its  diaphragm,  and  the  diaphragm  is,  there- 
fore, thrown  into  vibrations  which  correspond  with  the  current  waves. 
The  result  of  these  vibrations  of  the  second  diaphragm  is  to  send  out 
waves  of  sound  like  those  which  set  the  diaphragm  of  the  first  telephone 
to  vibrating. 

309.  Telephone  Transmitters. — The  original  Bell  telephone  is  not 
sufficiently  powerful  as  a  transmitter  to  give  satisfactory  service,  but  it  is 
an  extremely  sensitive  and  satisfactory  receiver.  The  transmitters  which 
are  now  generally  used  are,  therefore,  based  on  an  entirely  different 
principle. 

When  two  bits  of  carbon  are  permitted  to  lie  loosely  against  each 
other,  the  electrical  resistance  of  their  contact  is  very  much  changed 
when  changes  occur  in  the  pressure  of  the  contact ;  and  also  if  a  blunt 
metal  point  lies  loosely  against  the  carbon  surface,  differences  of  pressure 
at  the  contact  cause  variations  in  its  resistance.  Advantage  is  taken  of 
this  principle  in  the  common  telephone  transmitter  known  as  the  Blake 
transmitter.  Figure  250  is  a  diagram  of  such  a  transmitter.  M  is  a 
1  Articles  137,  138,  and  140. 


THE  TELEPHONE 


355 


phone  Telephone  Trans- 
mitter. 


mouthpiece,  and  D  is  the  diaphragm.  Touching  the  back  of  the  dia- 
phragm is  a  piece  of  platinum  wire,  /,  about  73¥  inch  in  diameter  and  \ 
inch  long,  which  is  soldered  into  a  hole  at  the  end  of  a  very  fine  German 
silver  spring,  x.  The  other  end  of  this  piece  of  platinum  makes  a  loose 
contact  at  C  with  the  polished  face  of  a  carbon  button  which  is  sus- 
pended on  a  piece  of  very  flexible  watch  spring,  y. 
The  amount  of  pressure  at  the  contact  is  exceed- 
ingly small  and  may  be  very  delicately  adjusted 
by  the  screw  which  is  shown  near  the  bottom  of 
the  figure. 

Platinum  and  carbon  electrodes  are  used  in 
this  transmitter  in  preference  to  two  carbon  elec- 
trodes because  there  is  less  sparking  between 
them  than  there  would  be  between  two  carbon 
surfaces,  and  the  conductivity  of  the  surfaces  is   FIG.  250.  —  Diagram  of  the 
preserved  for  a  longer  time.     The  carbon  button      Commonly  Used  Micro- 
and  platinum  piece  are  represented  by  the  two 
heavy  black  spots  at  C  in  the  figure. 

If  the  diaphragm  of  this  transmitter  (which  is  commonly  known  as 
the  Blake  transmitter)  is  spoken  to,  it  vibrates  and  causes  the  platinum 
point  to  press  more  or  less  lightly  upon  the  carbon  button  and  thus 
varies  the  resistance  of  the  contact.  If  the  transmitter  is  connected  in 
a  circuit  including  a  battery  and  Bell  telephone  receiver,  as  shown  in 
Figure  250,  the  current  flowing  in  the  circuit  varies  with  the  resistance  of 
the  carbon  contact  when  the  transmitter  diaphragm  vibrates.  The 
current  in  the  circuit  is,  therefore,  thrown  into  waves  which  correspond 
with  the  vibrations  of  the  diaphragm.  As  these  waves  of  current  pass 
through  the  coil  of  the  receiver,  they  increase  and  decrease  the  strength 
of  the  magnet,  and  its  diaphragm  is  thrown  into  vibration  so  that  the 
original  sounds  are  reproduced,  as  already  explained  in  Article  308. 

A  transmitter  in  which  two  carbon  electrodes  are  separated  by 
granules  of  carbon  is  also  commonly  used.  Such  a  one  is  illustrated 
in  Figure  259. 

310.  Microphones.  —  Such  a  carbon  contact  as  is  used  in  a  tele- 
phone transmitter  is  called  a  Microphone,  and  a  transmitter  in  which  it 
is  used  is  often  called  a  Microphone  Transmitter.  A  very  easily  made 


356 


ELECTRICITY   AND   MAGNETISM 


microphone,  in  which  both  electrodes  are  of  carbon,  is  shown  in  Figure 

251.     In  this  figure,  C  represents  a  short  stick  of  carbon  with  pointed 

ends,  which  is  held  loosely  between  the 
carbon  blocks  BB.  These  blocks  are  a 
little  countersunk  so  as  to  keep  the  carbon 
stick  from  falling  out,  and  they  are  fastened 
to  a  thin  piece  of  pine  board,  .S1.  The 
whole  may  be  mounted  on  a  board.  When 
this  microphone  is  connected  in  circuit 
with  a  cell  of  battery,  B,  and  a  telephone 
receiver,  R,  by  means  of  the  wires  which 
are  attached  to  the  carbon  blocks,  it  will 
transmit  sounds  to  the  telephone.  Such 
a  rough  microphone  will  not  transmit 

speech  so  that  it  can  be  understood,  but  it  will  cause  the  telephone  to 

soun  1  for  the  slightest  whisper. 

The  credit  of  the  invention  belongs  jointly  to  Professor  D.  E.  Hughes, 

of  Great  Britain,  and  to  Edison.    Each  discovered  the  properties  of  loose 


FIG.  251. —  Diagram  of  a  Simple 
Form  of  Microphone. 


£RMINALS  OF  PRIM 

WINDING 


TERMINALS  OF  SE( 
WINDING 


E  OF  IRON  WK 


HEAD  OF  INSULATING 
MATERIAL 


TUBE  OF  INSULATING. 
MATERIAL 


TERMINALS  OF  .PRIM 

WINDING 


SECONDARY  WINDIN3 


PRIMARY  WINDING 


ERMINALS  OF  stooi1 

WINDING 


-CORE  OF  IRON  WIRE 


FIG.  252.  — Perspective  View  and  Lengthwise  Cross  Section  of  Telephone  Induction  Coil. 

carbon   contacts  in    1878.     The   form  of  microphone   constructed  by 
Hughes  was  strictly  a  loose  contact  device  such  as  is  illustrated  in  Fig- 


THE  TELEPHONE 


357 


ure  251,  while  that  invented  by  Edison  consisted  of  a  soft  mass  of  lamp 
black  between  two  electrodes,  one  of  which  was  attached  to  a  telephone 
transmitter  diaphragm. 

311.  The  Induction    Coil.  —  The   ordinary   microphone    transmitter 
which  is  used  in  telephony  is  a  rather  delicate  affair,  and  it  cannot  be 
worked  with  more  than  one  or  two  battery  cells.     In  order  that  long 
lines  may  be  satisfactorily  spoken  over,  the  effect  of  the  transmitter  is 
intensified  by  means  of  electromagnetic  induction.      The  induction  coiP 
(Fig.  252),  which  is  used  for  this  purpose,  also  has  the  advantage  of 
shutting  out  a  scratchy  sound  which  is  caused  by  the  transmitter.     The 
primary  winding  of  the  induction  coil  is  connected  in  series  with  the 
transmitter   and   battery   and    the   secondary    winding   in   series   with 
the  line. 

312.  Complete  Telephone  Set.  —  The  commercial  telephone   system 
consists  of  much  more  than  the  transmitter   and  receiver  with   their 
accompanying  battery  and  line.     When  telephones  are 

used  simply  to  connect  two  points,  there  must  be  lo- 
cated at  each  point  a  transmitter,  a  receiver,  a  battery 
cell,  a  means  of  operating  an  electric  call  bell  at  the 
other  point,  and  a  local  call  bell.  This  outfit  is  usually 
put  up  in  a  set  like  the  familiar  form  shown  in  Figure 
253.  Here  A  represents  the  transmitter,  B,  the  re- 
ceiver, C,  a  box  containing  the  battery  cells,  DD,  the 
electric  call  bells,  and  E,  a  box  containing  a  small 
dynamo  with  permanent  magnets  called  a  magneto,2 
which  may  be  operated  by  a  crank.  The  magnet  is 
used  for  operating  the  call  bells.  When  the  receiver 
is  not  in  use,  it  hangs  on  a  hook  (as  shown)  which  is 
depressed  by  the  weight  of  the  receiver  and  moves 
electrical  contacts  which  connect  the  bells  and  magneto 
into  the  circuit  and  disconnect  the  telephone  instru- 
ments. When  the  receiver  is  taken  from  the  hook,  the  latter  rises  so 
that  the  contacts  cut  the  bells  and  magneto  out  of  circuit  and  the  tele- 
phone instruments  into  the  circuit. 

Figure  254  shows  a  diagram  of  the  circuits  in  an  ordinary  commercial 

1  Article  139.  2  Article  196. 


FIG.   253. —  Tele- 
phone Set. 


358 


ELECTRICITY   AND    MAGNETISM 


set.     When  the  receiver  is  hung  on  the  hook,  the  end  of  the  hook  lever 
comes  in  contact  with  P,  thus  connecting  the  magneto  generator  and 
bell  into  the  line  wires  which  connect  with  the 
telephone  set  at  L  and  L'.     When  the  receiver 
is  removed  from  the   hook,   a  spring  lifts  the 
hook  and  brings  the  lever  into  contact  with  N 
and    O,  thus  connecting  the  secondary  of  the 
induction  coil  and  the  receiver  into  the  line  cir- 
cuit and  closing  the  battery  circuit  through  the 
|       transmitter.      The    method   of  using   the  tele- 
I   ^11  phone   is  too  well   known   to   require   descrip- 

tion. 

313.  Telephone  Exchanges.  —  The  use  of 
telephones  simply  to  connect  two  points  is  only 
a  small  part  of  the  field  of  usefulness  of  the 
telephone.  The  great  majority  of  telephones 
are  used  in  connection  with  a  Central  Exchange. 
This  is  a  place  where  many  telephone  lines  cen- 
tre and  are  brought  to  a  Switchboard  so  that 
they  may  be  readily  connected  with  each  other. 
By  this  arrangement  each  telephone  user  in  a 
great  city  may  have  his  telephone  quickly  con- 
nected with  that  of  any  other  person.  Each 
telephone  user  or  Subscriber  is  supplied  with 
a  set  such  as  is  shown  in  Figure  253,  and  his 

line  is  run  from  the  telephone  set  to  a  section  on  the  switchboard  at 
the  exchange  which  bears  the  subscriber's  individual  number.  When 
one  subscriber  wishes  to  speak  with  another,  he  turns  the  crank  of  his 
magneto,  thus  causing  a  signal  at  the  switchboard.  He  then  takes  his 
telephone  receiver  from  its  hook,  and  when  the  switchboard  attendant 
speaks,  he  asks  her  to  connect  him  with  the  number  of  the  second  sub- 
scriber. This  being  done,  the  attendant  rings  the  telephone  bell  of  the 
second  subscriber  by  means  of  a  magneto,  and  this  calls  him  to  the 
telephone.  When  the  conversation  between  the  two  subscribers  is 
completed,  one  of  them  notifies  the  switchboard  attendant  by  means 
of  his  magneto. 


FIG.  254.  —  Diagram  of  the 
Electrical  Connections 
in  a  Telephone  Set. 


THE  TELEPHONE 


359 


314.  Telephone  Switchboards.  —  Telephone  switchboards  are,  as  a 
rule,  quite  complicated,  since  an  exchange  is  always  connected  to  a 
large  number  of  wires,  and  since  the  connections  of  the  telephone  wires 
must  be  arranged  so  that  the  operators  and  the  subscribers  are  able  to 
signal  and  talk  to  each  other,  as  well  as  so  that  the  subscribers'  lines 
may  be  quickly  connected  together. 

In  the  earliei  and  simpler  forms  of  telephone  switchboards  the  sub- 
scribers' wires  on  entering  the  exchange  are  each  connected  to  a  switch- 
board circuit  which  contains  a  Spring  Jack  and  an  electromagnet  which 


CROP  SHUTTER  P,  IN  RAISED 
POSITION 


=J_^  ARMATURE,  A. 
AGNET  CORE 


ARMATURE,  A, 
MAGNET  CORE 


FIG.  255.  —  Electromagnet  and  Shutter  for  Telephone  Switchboard. 

controls  a  Drop  or  shutter.  The  circuit  terminates  at  a  ground  plate. 
One  form  of  the  electromagnet  with  its  drop  is  illustrated  in  Figure  255. 
The  armature,  A,  of  the  electromagnet,  M,  has  a  hook,  D,  which 
ordinarily  supports  the  shutter  or  drop,  P,  which  is  hinged  at  the 
bottom  in  a  vertical  position.  When  the  subscriber  sends  current  over 
the  line  from  his  magneto,  the  armature,  A,  is  attracted,  the  drop  is  re- 
leased and  falls  into  the  horizontal  position  illustrated  in  Figure  255,  and 
thereby  discloses  the  subscriber's  number  which  is  painted  at  its  back. 

A  "spring  jack"  is  an  arrangement  by  means  of  which  an  electrical 
connection  may  be  made  by  inserting  a  metallic  plug  into  a  hole  so  that 


360 


ELECTRICITY   AND   MAGNETISM 


it  touches  a  spring  which  is  in  electrical  connection  with  the  circuit. 
Figure  256  illustrates  a  spring  jack. 


FACE  OF  SWITCH  BOA^D 


FIG.  256.  — Telephone  Spring  Jack. 

When  a  subscriber  Calls  by  working  his  magneto,  the  fact  is  indicated 
at  the  exchange  by  the  fall  of  the  Drop  belonging  to  his  line.  The  ex- 
change operator  inserts  in  this  sub- 
scriber's spring  jack  a  plug  which  is 
at  one  end  of  a  conducting  cord,  and 
at  the  same  time  moves  a  switch 
which  connects  her  telephone  to  his 
line.  She  inquires  what  connection 
is  desired,  makes  it  by  inserting  the 
plug  at  the  other  end  of  the  con- 
ducting cord  in  the  spring  jack  be- 
longing to  the  line  of  the  desired 
subscriber,  and  calls  him  by  pushing 
a  button  which  causes  his  telephone 
bell  to  ring.  This  completes  the 
operator's  duty  in  connecting  the 
two  subscribers.  When  the  sub- 
scribers have  finished  talking,  one  of 
them  turns  his  magneto  crank,  which 
causes  a  special  "  drop  "  to  fall  in  the 
exchange  and  calls  the  operator's 
attention  to  the  fact  that  the  lines 
may  be  disconnected. 

FIG.  257.  —  Simple  Form  of  Telephone  ....  .  .  , 

Switchboard.  Boards  of  this  general  type  are  used 


THE  TELEPHONE  361 

in  nearly  all  small  exchanges  in  the  country.  One  is  shown  in  Figure 
257.  It  will  be  noticed  that  the  cords  are  held  down  under  the  table 
by  weights  running  on  pulleys.  This  keeps  the  cords  from  getting  tan- 
gled, and  the  plugs  are  held  in  a  convenient  position  for  the  operator  to 
pick  up.  The  cords  are  really  flexible  conductors  which  make  electrical 
connections  between  the  plugs  at  their  respective  ends. 

315.  Multiple  Boards.  — As  one  operator  can  take  care  of  the  calls 
from  only  a  limited  number  of  subscribers  (50  to  100  is  the  usual  num- 
ber per  operator),  a  great  many  boards  of  the  kind  described  would  be  re- 
quired in  the  larger  exchanges,  and  much  difficulty  and  waste  of  time  would 
be  experienced  in  making  connections  between  the  line  of  a  subscriber 
connected  to  one  board  and  the  line  of  a  subscriber  connected  to  another 
board  in  another  part  of  the  room.  Hence,  what  are  known  as  Multiple 
Switchboards  are  used  in  exchanges  having  many  subscribers. 

The  multiple  board  with  its  numerous  details  can  be  explained  here 
only  in  the  briefest  outline.  The  principle  upon  which  it  is  based  is  to 
divide  the  total  number  of  subscribers'  lines  into  sets,  each  of  which  is 
brought  to  a  different  section  of  the  switchboard  where  the  lines  belong- 
ing to  the  set  may  be  looked  after  by  an  operator.  The  lines  are  con- 
nected to  a  drop  and  a  spring  jack  in  their  proper  sections,  so  that  the 
operator  may  communicate  with  the  subscribers  by  means  of  her  tele- 
phone set.  In  addition  to  entering  its  own  section  through  a  drop  and 
spring  jack,  every  subscriber's  line  is  also  connected  to  a  spring  jack  in 
every  other  section.  Consequently  each  operator  attends  to  the  .calls 
of  a  limited  number  of  subscribers  whose  lines  are  connected  to  drops 
in  her  section,  and  since  all  other  lines  have  spring  jacks  in  her  section 
she  can  connect  any  of  her  subscribers'  lines  to  the  line  of  any  other 
subscriber  which  enters  the  exchange. 

Figure  258  shows  the  principle  of  the  multiple  board.  The  dots 
marked  "  local  jacks  "  in  each  section  represent  the  spring  jacks  belong- 
ing to  the  lines  which  are  looked  after  by  the  operator  at  the  section. 
The  drops,  the  keys  for  ringing  up  subscribers,  the  operator's  telephone 
set,  etc.,  are  omitted  from  the  figure  for  the  sake  of  simplicity.  The 
dots  marked  "  ordinary  jacks"  represent  the  multiple  spring  jacks,  by 
means  of  which  the  operator  may  connect  any  one  of  her  subscribers  with 
any  other  that  is  connected  with  the  exchange.  It  will  be  seen,  for 


362 


ELECTRICITY  AND   MAGNETISM 


instance,  that  subscribers'  lines,  Numbers  i,  2,  and  3,  enter  the  local 
jacks  of  section  Number  i,  but  they  also  enter  the  ordinary  jacks  of 
the  other  sections.  If  an  operator  in  the  second  section  wishes  to  con- 
nect one  of  her  wires,  say  Number  6,  with  one  of  those  belonging  to  the 
first  section,  say  Number  3,  she  is  able  to  do  so  at  once  on  her  part  of 

the    board,    as    shown 

SUBSCRIBERS  in    the    figure> 

An  ingenious  ar- 
rangement by  which 
the  operator  can  tell 
when  a  line  is  in 
use,  prevents  switch- 
ing three  subscribers 
together.  Exchanges 
with  multiple  switch- 
boards have  been 
planned  to  give  tele- 
phone service  to  the 
enormous  number  of 
ten  thousand  subscrib- 
ers from  one  board, 
but  the  telephone  ser- 
vice has  never  yet 
.  risen  in  any  place  to 

such  magnitude  that  any  exchange  has  reached  the  number  of  ten  thou- 
sand subscribers,  since  it  is  customary  to  divide  the  service  among  sub- 
stations. This  is  done  in  the  large  cities  for  the  purpose  of  economizing  in 
the  construction  of  lines,  and  several  sub-exchanges  are  therefore  located 
to  serve  the  districts  outside  of  the  area  immediately  around  the  main 
exchange.  This  practice  causes  the  number  of  subscribers'  lines  attached 
to  any  one  exchange  to  be  smaller  than  might  otherwise  be  expected. 

316.  Ground  Returns.  —  The  earth  has  in  the  past  been  very  com- 
monly used  as  one-half  of  telephone  circuits,  so  that  only  one  wire  need 
be  used  to  connect  an  instrument  with  the  exchange.  The  ground  ter- 
minals of  the  instruments  are  then  connected  by  wire  to  gas  or  water 
pipes,  or  to  iron  bars  driven  into  the  ground.  The  telephone  receiver 


SECTION  NO.  2  SECTION  NO.  3 

EXCHANGE 

FIG.  258.  —  Diagram  of  Multiple  Switchboard. 


THE  TELEPHONE 


363 


FIG.  259.  —  Cross  Section  of  a 
Long-distance  Telephone  Trans- 
mitter. 


is  such  an  exceedingly  delicate  instrument  that  outside  currents  are 
likely  to  affect  its  operation  when  the  Ground  Return  is  used,  and  impor- 
tant lines  are  nowadays  constructed  with 
a  complete  Metallic  Circuit ;  that  is,  with 
metal  wires  for  both  the  outgoing  and  in- 
coming part  of  the  line.  A  special  trans- 
mitter, called  the  long-distance  transmitter, 
is  generally  used  with  metallic  circuits. 
This  transmitter,  which  is  shown  in  Figure 
259,  is  a  microphone  transmitter  in  which 
the  loose  contact  between  a  bit  of  platinum 
and  a  carbon  button  is  replaced  by  a  short 
tube  faced  with  metal  or  carbon  buttons 
and  containing  powdered  carbon.  The 
carbon  "granules"  contained  in  the  tube 
are  so  arranged  that  the  vibrations  of  the 

diaphragm  vary  the  pressure  with  which  they  lie  against  each  other,  and 

the  total  resistance  of  the 
tube  passes  through  wide 
variations.  This  transmitter 
is,  therefore,  very  powerful, 
and  is  especially  useful  in 
connection  with  circuits  of 
great  length. 

317.  Simultaneous  Te- 
lephony and  Telegraphy.  — 
The  rapid  variations  of  the 


telephone  current,  which 
are  caused  by  the  vibrations 
of  the  diaphragm  under  the 
influence  of  the  voice  waves, 
transmit  themselves  by  in- 
duction through  a.  conden- 
ser, while  the  slow  make 
and  break  signals  of  the  Morse  telegraph  do  not.  Utilizing  these  char- 
acteristics makes  it  possible  to  adapt  a  circuit  for  telephoning  and 


TELEPHONE   SET 


FlG.  260.  —  Diagram  of  Circuits  for  Telegraphing 
and  Telephoning  simultaneously. 


364  ELECTRICITY  AND   MAGNETISM 

telegraphing  at  the  same  time.  This  double  use  of  telephone  con- 
ductors is  extensively  adopted  on  the  long-distance  telephone  lines  in 
this  country. 

A  diagrammatic  sketch  illustrating  one  arrangement  of  the  apparatus 
is  shown  in  Figure  260.  The  rapidly  varying  telephone  currents  are 
effectively  choked  back  by  the  high  self-induction  of  the  telegraph  in- 
struments, so  that  they  do  not  dribble  off  through  them  ;  and  the  con- 
densers in  circuit  with  the  telephones  keep  the  telegraph  signals  from 
interfering  with  the  talking  currents. 

318.  Wiring  Bell  Circuits.  — The  wires  used  for  electric  bell  circuits 
have  a  very  different  insulation  from  that  of  electric  light  wires.1 
The  wire  commonly  used  inside  of  buildings  for  bell  circuits  is  called 
"  annunciator  wire."  It  is  a  copper  wire  with  an  insulation  consisting  of 
two  heavy  cotton  wrappings,  wound  in  opposite  directions,  and  thor- 
oughly waxed  and  paraffined.  These  wires  are  made  of  various  sizes 
and  are  frequently  striped  in  different  colors.  Sometimes  what  is  known 
as  "office  wire"  is  used  for  telephone  and  messenger  call  connections. 
The  insulation  of  "  office  wire  "  ordinarily  consists  of  two  braidings  of 
cotton  which  are  well  soaked  in  paraffine. 

While  no  danger  can  arise  from  the  use  of  these  poorly  insulated  wires 
for  such  circuits,  provided  they  are  not  in  a  position  to  come  in  contact 
with  electric  light  wires,  yet  a  great  deal  of  inconvenience  is  caused  by 
their  unsatisfactory  and  leaky  character.  This  is  the  condition  of  num- 
berless electric  bell  circuits  in  houses  all  over  the  country  where  the 
front  door  bells  fail  to  ring  when  the  buttons  are  pushed.  The  trouble 
is  caused  by  the  current  leaking  from  poorly  insulated  wires  where  they 
come  in  contact  with  dampness  or  at  some  point  where  they  are  both 
placed  under  one  metal  staple,  and  the  difficulty  in  a  great  majority 
of  the  cases  would  never  have  appeared  had  wire  with  good  rubber 
insulation  been  used.  As  No.  18  B.  &  S.  wire  is  usually  used  for  the 
bell  circuits,  the  extra  cost  caused  by  using  rubber-covered  or  "  weather 
proof"  wire  is  not  very  great,  while  the  inconvenience  avoided  by  its 
use  may  be  considerable. 

It  must  not  be  assumed,  however,  that  all  the  troubles  to  which  bell 
circuits  and  similar  circuits  are  heir  arise  from  poor  insulation.  Battery 

i  See  Chapter  XX. 


ELECTRIC  BELLS 


365 


zincs  become  used  up  or  the  water  evaporates,  and  the  battery  may  not 
work  on  that  account.  The  mechanism  of  bells  and  push  buttons  is  very 
simple  and  not  likely  to  get  out  of  order,  but  trouble  may  occur  even  in 
them.  The  contact  in  a  push  button  gradually  becomes  corroded,  and 
then  when  the  button  is  pushed  it  does  not  complete  the  circuit.  This 
fault  is  easily  remedied  by  taking  the  cover  off  the  button  and  scraping 
the  contact  points. 

Figure  261  is  a  diagram  which  shows  two  arrangements  for  electric 
bell  circuits.      The  battery  consists  of  one  or  two  open  circuit  cells. 


DW9RAM   SHOWING  CIRCUIT  WITH  TWO  PUSH  BUTTONS  FOR  A  SINGLE_BELL 

_- - 

/       /^ 


DIAGRAM   SHOWING  CIRCUIT  FOR  RINGING  TWO.  BELLS  FROM  ONE  PUSH   BUTTOH 

FIG.  261.  —  Diagram  of  Electric  Bell  Circuits. 

These  are  connected  in  series  with  the  bell  and  push  button  by  wires 
which  may  run  within  the  walls  of  a  house.  When  the  button  is  pushed, 
it  closes  the  circuit  and  the  bell  rings.  When  the  button  is  not  being 
pushed,  the  circuit  should  be  open  and  the  battery  at  rest. 

If  a  leak  occurs  from  wire  to  wire,  the  battery  remains  in  action  all  the 
time,  and  the  depolarizer l  (if  the  battery  has  one)  soon  becomes  ex- 
hausted and  the  battery  becomes  polarized  or  "run  down."  .The  bell 
then  fails  to  ring  when  the  button  is  pushed.  If  the  battery  has  no 
depolarizer,  the  process  of  running  down  occurs  in  exactly  the  same  way, 
but  it  is  more  rapid. 

1  Article  41. 


366 


ELECTRICITY   AND    MAGNETISM 


FIG.  262.  —  Diagram  of  Household  Bell  Circuit. 


When  one  bell  is  operated  from  one  push  button,  the  circuit  is  exactly 
the  same  as  though  one  push  button  were  removed  from  Figure  261. 

The  bell  service  in 
most  houses  is  per- 
formed by  several 
circuits,  each  of 
which  includes  a 
bell,  B,  and  a  push 
button,  A  ;  but  one 
battery  is  used  in 
common  for  all  cir- 
cuits. This  is  il- 
lustrated in  Figure 
262,  from  which  it 
may  be  seen  that 
the  bell  circuits  are 
connected  in  par- 
allel to  the  com- 
mon battery,  C. 

319.  Electric  Bells. — The  mechanism  of  the  ordinary  vibrating  elec- 
tric bell  consists  of  a  stationary  electromagnet,  E  (Figure  263),  with  a 
vibrating  armature,  A,  which  is  fastened  at 
one  end  to  a  spring  hinge,  S,  and  carries 
at  the  other  end  the  bell  clapper,  H. 
When  an  electric  current  is  passed  through 
the  electromagnet  of  a  bell,  the  armature 
is  attracted  and  moves  forward  so  that  the 
clapper  strikes  the  gong.  At  the  same 
time  the  electric  circuit  is  broken  by  a 
spring  contact,  C,  at  the  back  of  the  arma- 
ture, the  magnet  loses  its  magnetism,  and 
the  armature  flies  back  to  its  original  posi- 
tion. When  the  armature  flies  back,  the 
circuit  is  again  completed  at  the  spring 

contact,  C,  the  armature  again  flies  forward,  the  clapper  again  strikes  the 
gong,  and  the  whole  process  is  rapidly  repeated  over  and  over  again  as 


FIG.  263.  —  Electric  Bell  with  Bat- 
tery and  Push  Button. 


ELECTRIC   BELLS  367 

long  as  the  electric  circuit  is  complete  at  the  push  button.  The  back- 
and-forth  motion  of  the  armature  causes  the  clapper  to  strike  a  succes- 
sion of  blows  on  the  gong,  and  thus  causes  the  ringing  of  the  bell.  When 
a  bell  gets  out  of  order,  the  trouble  is  usually  to  be  found  in  the  spring 
contact,  which  may  be  dirty  or  out  of  adjustment,  or  the  electromagnets 
may  be  short-circuited. 

Figure  264  is  a  diagram  of  a  push  button, 
the  simplicity  of  which  is  to  be  seen  at    a 

glance. 

~  .      .  i-,i  ,  F1G-  264.  — Cross  Section  of 

Sometimes  it  is  not  desirable  to  have  an        Ordinary  Push  Button. 

electric  bell  ring  continuously  while  the  push 

button  is  depressed,  and  in  that  case  the  spring  contact,  S,  is  omitted, 
and  the  conducting  wires  are  connected  directly  to  the  electromagnet. 
Such  a  bell  makes  a  single  stroke  for  each  push  of  the  button,  and  it  is 
therefore  often  called  a  "  single-stroke  bell." 

It  is  an  interesting  fact  that  the  use  of  electric  bells  was  the  first  appli- 
cation of  electricity  to  household  purposes,  and  that  the  principle  of  the 
electric  bell  was  first  made  use  of  by  Professor  Joseph  Henry  about  1830. 

QUESTIONS 

38.  What  do  the  words  "  telegraph  "  and  "  telephone  "  mean? 

39.  When  was  the  first  speaking  telephone  invented?     By  whom? 

40.  Describe  a  Bell  telephone. 

41.  What  causes  sounds  to  be  heard  in  one  Bell  telephone  which  are  similar  to 
those  spoken  to  a  second  telephone  properly  connected  with  the  first? 

42.  Is  a  permanent  magnet  essential  to  the  working  of  a  Bell  telephone? 

43.  Why  is  the  Bell  telephone  not  commonly  used  as  a  transmitter? 

44.  How  may  the  resistance  of  loose  carbon  contacts  be  varied? 

45.  What  is  the  principle  of  the  Blake  transmitter? 

46.  Why  does  a  Bell  receiver  properly  placed  in  circuit  reproduce  words  spoken 
into  a  Blake  transmitter? 

47.  Who  discovered  the  properties  of  loose  carbon  contacts? 

48.  What  is  a  microphone? 

49.  How  does  a  Blake  transmitter  differ  from  an  ordinary  microphone? 

50.  Why  is  a  transformer  or  induction  coil  used  with  a  carbon  transmitter? 

51.  What  apparatus  is  placed  in  telephone  sets? 

52.  What  is  the  purpose  of  each  part  of  the  set? 

53.  Describe  the  circuits  of  a  telephone  set. 


368  ELECTRICITY  AND   MAGNETISM 

54.  How  are  the  circuits  switched  on  to  and  off  the  line? 

55.  What  is  a  central  telephone  exchange? 

56.  What  is  the  purpose  of  a  telephone  switchboard? 

57.  What  are  the  "  drops  "  on  a  telephone  switchboard  for?     How  do  they  work? 

58.  How  does  a  multiple  switchboard  work? 

59.  Why  are  metallic  circuits  preferable  for  telephone  work? 

60.  What  is  the  essential  difference  between  a  long  distance  and  a  Blake  trans- 
mitter ? 

61.  How  may  telephony  and  telegraphy  be  carried  on  over  the  same  wires? 

62.  How  is  an  electric  bell  made? 

63.  How  does  an  electric  bell  work? 

64.  When  did  Henry  make  and  use  the  first  electric  bell? 

65.  How  are  bells  connected  up  for  service? 

66.  What  kinds  of  wire  are  used  in  bell  circuits? 

67.  Why  must  bell  wires  be  well  insulated? 

68.  What  troubles  are  apt  to  appear  in  bell  circuits? 


CHAPTER   XX 


LINE  CONSTRUCTION  AND   THE   ELECTRIC   DISTRIBUTION  AND 
TRANSMISSION    OF   POWER 

320.  Line  Material.  —  A  large  proportion  of  the  electric  light  and 
power,  telegraph  and  telephone  lines  in  the  United  States  are  supported 
on  the  wooden  poles  which  are  so  common  in  city  streets  and  along 
railroads  and  highways.  Usually  these  poles  are  made  of  white 
cedar  or  chestnut,  though  in  some  parts  of  the  country  pine,  cypress, 
and  tamarack,  or  other  woods,  are  used.  The  poles  differ  in  length 
from  25  feet  to  as  much  as  100  feet,  and  in  diameter  at  the  upper 
end  from  4  or  5  inches  to  8  or  10  inches.  The  sizes  which  are 
most  commonly  used  are  from  25  to  60  feet  long  and  from  6  to 
8  inches  in  diameter  at  the  top.  These  are  set  in  holes  dug  in  the 
ground  to  a  depth  which  depends  upon  the  length  of  the  poles  and 
the  importance  of  the  lines  which  they  carry,  but  which  is  approxi- 
mately equal  to  one-sixth  of  the  length  of  each  pole. 
The  wires  which  are  supported  by  the  poles  are  usu- 
ally tied  fast  to  glass  Insulators,  which  are  screwed 
on  oak  or  locust  Pins  (Fig.  265),  fastened  to  pine 
Cross  Arms.  The  arms  are  usually  3^  x  4^  inches, 
and  as  long  as  required.  Figure  266  shows  the  gen- 
eral arrangement  of  a  line  of  18  wires.  The  cross 
arms  fit  into  notches  called  Gains,  which  are  cut  in 
the  sides  of  the  poles,  and  they  are  then  fastened  in 
place  by  means  of  lag  screws  or  bolts. 


GLASS  INSULATOR 


FIG.  265.  —  Wood 
Pin  with  "  Pony  " 


Poles  are   commonly  erected  with  the  cross  arms      Class    Telephone 

f     .  ,         ,  ,  •          r         i         /T--  Insulator,      about 

facing  each  other  on  alternate   pairs  of  poles   (Fig.       one-seventh  size. 

267).     If  they  are  set  in  this  manner,  the  arms  are 
not  likely  to  be  pulled  off;  but  when  all  the  arms  face  in  one  direc- 
tion, it  is  possible  for  all  of  them  to  be  pulled  off,  one  after  another, 

2B  369 


370 


ELECTRICITY  AND   MAGNETISM 


on  account  of  the  breaking  of  a  pole  or  of  one  of  the  arms.     Figure  268 
shows  the  top  of  a  pole  arranged  to  carry  50  long-distance  telephone 

wires.  The  numbers  marked  on  the 
figure  show  the  dimensions.  In  this 
figure  the  cross  arms  are  shown  braced 
with  iron  braces.  These  are  ordinarily 
used  when  the  arms  are  intended  to 
carry  heavy  or  particularly  important 
wires.  The  braces  are  quite  commonly 
used  in  cities,  but  are  omitted  in  coun- 
try telegraph  lines.  Iron  or  steel  wires 
are  very  commonly  used  for  telegraph  and  telephone  lines,  but  copper 
wires  are  used  on  the  most  important  long-distance  lines. 


FlG.  266.  —  Illustration  of  a  Short  Por- 
tion of  Pole  Line  carrying  Eighteen 
Telegraph  Wires. 


_i 

CF 

OSS  ARM                                         fC 

T 

ROS 

POLE-<I 

3*  —  POLES  —  <; 

ir12ir12ir12ir12s 


t==^ 


FlG.  267.  —  View  of  Pole  Line,  with  "  6  Pin  Cross  Arms,"  from  above. 

As  a  general  rule  electric  light  and  power  lines  carry  fewer  but  much 
heavier  wires  than  telephone  or  telegraph  lines.  The  sizes  of  the  wires 
depend  upon  the  current  transmitted 
over  them,  their  length,  and  the  drop 
of  pressure  which  is  permitted  to  occur 
in  them.  The  dimensions  are  deter- 
mined by  methods  to  be  explained.1 
Electric  light  and  power  wires  are  al- 
ways of  copper  or  aluminum  of  the 
highest  obtainable  conductivity,  and 
they  ordinarily  vary  in  size  from  No.  8 
to  No.  oooo  B.  &  S.  gauge,  or  from 
about  4-  of  an  inch  in  diameter  to 

o 

nearly  i  of  an  inch  in  diameter.     The 

former  is  the  smallest  wire  of  soft  cop-   FlG"  268-~  Cross  Arms-  Pins-  and 

per  which  can  be  depended  upon  not 


9    8    8 


^ 


-% 


9 


^ 


7? 
i 


// 

/       s — INSULATORS 
S       g       g       g 


2s 

/IRO 


ARM 
IRON  BRACE 


T  T 

Insulators  on  Long-distance  Tele- 

phone  Pole  arranged  forFifty  Wires. 


1  Article  329.    Also  Chapter  VII. 


LINE  CONSTRUCTION  371 

to  break  from  mechanical  strains  caused  by  its  swaying  in  the  wind, 
other  wires  falling  upon  it,  etc.,  while  the  latter  is  the  largest  solid 
wire  which  can  be  conveniently  handled ;  and  if  larger  conductors  are 
used,  they  are  made  out  of  strands  of  smaller  wires.  Where  a  number 
of  large  wires  are  run  on  the  same  pole  line,  extra  heavy  poles  and 
cross  arms  are  used. 

321.  Wire  Insulation.  —  The  wires  used  upon  overhead  telegraph  and 
telephone  lines  are  not  covered  with  insulation,  and  the  same  is  true  of 
some  low-pressure  electric  light  lines.  For  instance,  the  overhead  wires 
used  to  distribute  current  for  incandescent  lighting  by  the  ordinary 
three-wire  system  were  formerly  almost  always  bare,  and  the  glass  bells  at 
the  points  of  support  were  depended  onto  give  a  satisfactory  insulation. 
This  is  perfectly  safe  when  the  pressure  is  as  low  as  in  the  ordinary 
three-wire  system,  where  the  pressure  between  the  positive  and  negative 
wires  is  seldom  higher  than  260  volts.  When  the  pressure  used  on 
overhead  lines  is  higher  than  300  volts,  it  is  usual  to  use  insulated  wires 
within  the  limits  of  cities  and  towns.  The  insulation  consists  of  a  con- 
tinuous braided  cotton  covering  of  two  or  three  thicknesses,  which  is 
thoroughly  soaked  with  some  insulating  compound.  As  the  insulation  is 
supposed  to  be  partially  waterproof,  such  wire  is  often  called  Weather- 
proof Wire.  The  insulating  compound  is  almost  always  black. 

Black  weather-proof  wire  is  used  for  the  overhead  lines  of  power 
plants  which  distribute  current  to  motors  at  a  pressure  of  500  volts,  for 
the  overhead  lines  of  alternating  current  electric  light  plants,  which  use 
a  pressure  of  1000  or  2000  volts,  for  the  feeders  of  electric  railway  plants, 
arc-light  wires,  etc.  It  has  become  an  almost  universal  custom  in  this 
country  to  use  No.  6  B.  &  S.  gauge  weather-proof  wires  for  overhead 
arc-light  lines.  As  the  arc  current  seldom  exceeds  10  amperes,  the  loss 
of  pressure  in  a  No.  6  wire  several  miles  in  length  is  not  very  great,  and 
it  is  a  convenient  and  economical  size  to  use. 

The  circuits  for  electric  lighting  and  power  are  always  complete  wire 
circuits,  as  the  use  of  the  ground  for  returning  large  currents  is  sure  to 
cause  difficulties  from  the  uncertain  and  comparatively  large  resistance 
of  ground  plates,  and  a  grounded  electric  light  circuit  always  introduces  a 
risk  of  fire  in  each  house  that  it  enters*  For  the  latter  reason  fire  insur- 
ance men  or  Underwriters  refuse  to  approve  the  use  of  a  ground  return 


372 


ELECTRICITY  AND   MAGNETISM 


TIMBER  BRACE    // 


FlG.  269.  —  Corner  Pole 
braced  against  Side 
Strain. 


for  the  distribution  of  electric  light  and  power  where  the  wires  enter 
buildings  insured  by  them. 

322.  Erection  of  Poles.  —  In  erecting  the  poles,  care  must  be  taken 
that  they  are  set  in  straight  lines  as  much  as  possible.     If  curves  or  cor- 
ners  are  turned,   the  poles  at  the  turn   must  be 
Braced  or  Guyed  to  prevent  their  being  pulled  over 
by  the  strain  of  the  wires.      Figure  269  shows  a 
pole  which  is  braced  against  a  side  strain  by  means 
of  a  timber  brace,  and  Figure  2  70  shows  poles  held 
against  side  strains  by  wire  Guys  which  are  fastened 
in  two  ways.     When  the  guy  crosses   a  street  or 
other  passageway,  it  is  not  uncommon  to  make  the 
post  or  Stub  to  which  the  guy  is  attached  eight  or 

ten  feet  high.    The  latter  figure  shows  the  poles  somewhat  tipped  or 

inclined.     This  is  an  additional  safeguard  against  the  poles  being  pulled 

over  by  the  strain  of  the  wires.     In  cities  the  poles  are  ordinarily  shaved 

all  over  with  a  draw  knife  before  being  set,  and 

they  are  then  painted,  but  in  the  country  this 

refinement  is  not   generally  considered   to  be 

necessary.     The  distance  between  poles  which 

carry  telegraph  and  telephone  wires  varies  from 

about  1 20  feet  to  300  feet,  or  the  number  of 

poles  to  the  mile  varies  from  45  to  18.     Only 

in  the   case  of  important  lines  carrying  many 

wires  are  the  shorter  distances  between  poles 

used,  and  the  average  number  of  poles  used  for  lines  running  through 

the  country  is  from  20  to  30  per  mile.     Poles  which  carry  electric  light 

wires  are  generally  placed  at  shorter  distances   apart,  these  distances 

varying  from   100  feet  to  150  feet,  depending  upon  the  weight  of  the 

individual  wires  and  the  importance  of  the  lines. 

323.  Stringing  Wires.  —  Various  methods  are  used  in  stringing  wires. 
One  of  the  commonest  ways  is  to  unreel  a  certain  length  of  the  wire  out 
on  the  ground,  after  which  it  is  carried  to  its  place  on  the  insulators, 
drawn  up  tight,  and  tied  fast  by  linemen  who  climb  the  poles  by  means  of 
spurs  ;   and  then  the  operation  is  repeated  on  another  length.    In  another 
method,  the  set  of  wires  may  be  drawn  over  the  cross  arms  from  a  fixed  reel. 


FIG.  270.  —  Corner  Poles 
held  against  Side  Strain 
by  Wire  Guys. 


LINE  CONSTRUCTION 


373 


TIE  WIRE  WRAPPED 
AROUND  LINE  WIRE 


Wire  which  is  intended  for  use  on  pole  lines  is  usually  furnished  in 
coils  which  may  be  laid  on  a  hand  reel  or  a  reel  mounted  upon  a  wagon. 
When  a  considerable  length  of  wire  has  been  laid  on  the  cross  arms,  a 
block  and  tackle  is  attached  to  its  end  and  it  is  stretched  up  tight. 
While  the  wire  is  held  tight,  it  is 
tied  by  linemen  to  the  insulator 
upon  which  it  is  placed  at  each 
cioss  arm.  .  The  tie  is  usually 
made  of  wire  like  that  in  the 
lines,  but  it  is  often  somewhat 
smaller  in  size.  The  line  wire  FIG.  271.  — illustration  of  Loop  of  Tie  Wire 

is    laid   in    the    groove    of    an    in-  with  its  Ends  wrapped  tightly  around  Line 

\Virc 

sulator,  and  one  end  of  the  tie  m 

wire  is  twisted  tightly  around  it  close  to  the  insulator.  The  tie  wire  is 
then  carried  around  the  groove  in  the  insulator,  and  its  other  end  is 
twisted  tightly  around  the  line  wire  close  to  the  insulator.  The  appear- 
ance of  the  line  wire  and  loop  of  tie  wire  with  the  insulator  removed, 
is  shown  in  Figure  271. 

324.  Wire  Joints.  —  Wire  is  furnished  from  the  wire  mills  in  coils 
which  contain  lengths  varying  from  a  few  hundred  feet  to  a  half  mile 

or  more.     A  great  many  joints  are 

therefore  necessary  in  a  long  line_ 

The  commonest  form  of  joint  used 
in  this  country  is  that  known  as  the 
FIG.  272.  -  illustrations  of  "  Western  Un-    Western  Union  or  twist  joint,  which 

•  ion  Joints  "  in  Small  and  Medium-size  is  shown  in  Figure  272.  The  figure 

shows  the  way  a  twist  joint  appears 

when  made  of  iron  wire  of  medium  size  and  also  when  made  of  small 
copper  wire. 

In  electric  light  and  power  lines  these  joints  are  always  soldered,  in 
order  that  their  electrical  conductivity  may  be  as  great  as  possible,  and 
that  the  wires  at  the  joint  may  not  be  corroded  by  the  effect  of  gases 
which  are  in  the  air.  The  soldering  is  done  by  dipping  the  twisted  joint 
into  a  pot  of  melted  solder,  or,  if  the  wire  is  large,  by  pouring  solder  on 
the  joint  from  a  ladle.  Many  specially  arranged  "sleeve"  joints  are 
also  used.  Here  the  wires  are  slipped  from  opposite  directions  into  the 


374 


ELECTRICITY  AND    MAGNETISM 


sleeve,  and  the  sleeve  with  the  wires  is  thoroughly  twisted,  or  the  ends  of 
the  wire  are  pushed  beyond  the  ends  of  the  sleeve  and  are  turned  up  to 
avoid  their  pulling  out,  and  the  whole  joint  may  then  be  filled  with  solder. 
325.  Insulators.  —  The  insulation  of  all  kinds  of  electric  lines  is  a 
matter  of  much  importance.  The  effect  of  poor  insulation  is  illustrated 

in  Figure  273,  where  A 
and  B  are  two  telegraph 
stations  connected  by  a 
wire  strung  upon  the  poles 
marked  P,  P,  P,  P.  The 


*      1 

\  \ 

i 

i 

! 
i          i 

i 

/ 
j 

/ 

/ 

m 

FIG.  273.  —  Illustration  of  tile  Effect  of  Poor  Insulation. 


electrical  circuit  through 
the  stations  is  completed 
by  a  ground  return  through 
the  ground  plates  marked 

G,  G.     All  the  battery  is  supposed  to  be  located  at  station  A.      If 

the  wire  is  not  well  insulated  at  each  point  where  it  is  supported  at 

the   poles,   an   appreciable   portion   of  the   current   leaks   out  of  the 

line  into  the  earth  without  having  passed  through  the  distant  station. 

Each  point  of  leakage  gives  a  branch  circuit ;  and  if  the  line  is  long,  so 

that  many  such  branch  circuits  are  in  parallel,  the 

total  leakage  may  be  so  great  that  sufficient  current 

does  not  reach  station  B  to-  work  its  relay  when 

signals  are  made  by  means  of  the  key  at  station  A. 

The  actual  proportion  of  the  current  which  escapes 

by  leakage  depends  upon  the  ratio  which  the  line 

resistance  bears  to  the  combined  resistance  of  all 

the   leakage   paths   taken   in   parallel.1     Where  a 

Metallic  Circuit  (that  is,  where  wires  are  used  for 

both  outgoing  and  return  conductors)  is  used,  the 

leakage  paths  reach  from  one  wire   to  the   other 

instead  of  from  one  conductor  to  the  earth. 

To  make  the  resistance  of  the  leakage  paths  as 

great  as  possible,  the  insulators  to  which  the  line 

and  wire  are  attached  at  the  poles  are  commonly  made  of  glass  in  the 

form  of  bells  (Fig.  274),  which  may  be  screwed  on  the  wooden  pins  in 

1  Article  103. 


FIG.  274.  —  Glass  Tel- 
egraph Insulator. 
About  one-third 
size. 


LINE  CONSTRUCTION 


375 


the  cross  arms.  These  insulators  are  made  in  various  slightly  different 
forms  and  sizes.  That  shown  in  Figure  2  74  is  suitable  for  small  wires 
such  as  are  used  in  telegraphy  and  telephony.  That  shown  in  Figure 
275  is  suitable  for  the  larger  wires  used  in  electric  light  lines. 

Glass  is  an  excellent  insulator  when  it  is  dry,1  but  in  damp  weather  its 
surface  becomes  covered  with  a  thin  layer  or  film  of  water.  This  film  of 
water  makes  a  path  through  which,  at  every  insu- 
lator, a  small  portion  of  current  may  leak  from  the 
wire  to  the  wooden  supporting  pin,  and  thence  over 
the  damp  wood  of  the  cross  arm  and  pole  to  the 
ground,  or  to  some  other  wire.  Water  is  a  com- 
paratively poor  conductor,  and  the  quantity  of  cur- 
rent which  escapes  at  each  insulator  is  very  small, 
but  the  total  loss  at  all  the  insulators,  on  a  line 
several  hundred  miles  long,  may  be  a  very  serious 
matter.  As  the  leakage  at  each  insulator  is  along 
the  film  of  water  which  covers  the  surface  of  the 
glass,  the  effective  insulation  is  increased  by  in- 
creasing the  length  of  the  path  over  which  the 
current  must  pass  on  the  insulator's  surface.  This  is  done  by  adding  a 
second  "petticoat "  or  bell  to  the  glass,  as  shown  in  Figure  275. 

Rubber  hook  insulators  (Fig.  276)  which  may  be 
screwed  into  the  bottoms  of  cross  arms  or  in  other 
similar  positions  are  sometimes  used  for  special  work. 
In  Europe,  porcelain  bell  insulators  which  are  quite 
similar  in  shape  to  the  American  glass  insulators,  are 
used.  The  dense  white  porcelain  of  which  these  in- 
sulators are  made  is  in  some  respects  better  than  glass 
for  the  purpose,  but  it  is  more  expensive.  Large  por- 
celain insulators  are  also  being  used  in  this  country 
for  some  of  the  lines  over  which  power  is  transmitted 
by  the  electric  current  at  high  pressures.  Sometimes 
these  have  triple  petticoats. 
Where  telegraph  or  telephone  lines  run  inside  of  buildings,  it  is  usual 
to  use  copper  wire  of  a  small  size  which  is  insulated  by  a  braided  or 

i  Article  7. 


FIG.  275.  —  Glass  Insu- 
lator with  Double 
Petticoat.  About 
one-third  size. 


METAL  HOOK 

FIG.  270.— Rubber 
Hook  Insulator. 
About  one-fifth 
size. 


376 


ELECTRICITY  AND   MAGNETISM 


SCREW  THREAD 


wrapped  covering  of  cotton  thread  thoroughly  soaked  with  paraffine ; 
and  rubber  insulated  wire  is  used  for  electric  lighting  and  power  lines 
when  they  run  inside  of  buildings.  Where  single  lines 
need  support,  as  in  passing  from  the  poles  to  a  building, 
it  is  usual  to  use  an  oak  bracket  (Fig.  277)  with  a  glass 
insulator,  or  a  porcelain  knob  (Fig.  278)  fastened  at 
some  convenient  point. 

326.  Underground  Wires.  —  In  large  cities,  electric 
conductors  are  often  put  underground,  and  in  some 
places  they  are  run  over  housetops.  When  electric 
wires  are  placed  underground,  they  must  be  continuously 
insulated  by  some  material  which  is  sufficiently  flexible 
to  permit  the  wires  to  be  easily  handled.  For  power 
and  lighting  wires  this  insulation  often  consists  of  a 
thickness  of  a  vulcanized  rubber  compound  which  has 
been  placed  on  the  wire  under  hydraulic  pressure,  but 
the  covering  for  some  cables  is  made  by  closely  wrapping 
the  conductor  with  strips  of  paper  which  have  been 
soaked  in  an  insulating  compound  so  as  to  make  it 
quite  soft  and  flexible.  The  thickness  of  this  paper 
wrapping  is  made  about  the  same  as  that  of  rubber  insulation,  and  over 
it  is  put  a  lead  sheathing  which  is  similar  to  the  sheathing  of  rubber 
insulated  cables.  A  third  style  of  insulation  con- 
sists of  a  thick  braiding  or  wrapping  made  up  of 
several  layers  of  cotton  or  jute  which  is  soaked  in 
an  insulating  compound  quite  similar  to  that  used 
for  weather-proof  wires.  This  is  also  covered  with 
a  lead  sheathing.  The  latter  cables  are  often  said 
to  have  fibrous  insulations  on  account  of  the  char- 
acter of  the  materials  used.  As  fibrous  material 
will  rapidly  absorb  moisture  and  its  insulating  quali- 
ties are  then  ruined,  it  is  necessary  that  the  lead 
sheathing  shall  contain  no  holes,  however  small, 
and  the  ends  of  the  cables  must  be  carefully  protected  from  moisture. 
The  protection  of  rubber  insulation  is  not  so  important,  but  moisture  may 
even  here  have  a  serious  effect,  and  the  most  careful  handling  of  the 
cables  is  advisable. 


FIG.  277.  —  Oak 
Bracket  with 
Screw  Thread 
for  Glass  In- 
sulator. 


FIG.   278.  —  Porcelain 
Knob  Insulator. 


LINE  CONSTRUCTION 


377 


In  the  case  of  telephone  wires  it  is  particularly  important  that  their 
electrostatic  capacity  be  the  smallest  that  is  possible,  on  account  of 
the  delicacy  of  the  telephone 
current,  and   crinkled  paper 
is  often  used  for  their  insu- 
lation.1    When  a  fibrous  in- 
sulation   such    as    paper    is 

FIG.   270.  —  Electric   Light   Cable  with  a  Single 

used,  it  is  necessary  to  pro-  Conductor. 

tect  it  from  absorbing  mois- 
ture, and  a  lead  covering  over  the  insulation  is,  therefore,  used.    In  fact, 
the  lead  covering  is  generally  used  with  rubber  covered  wires  also,  in 

order  that  the  rubber  may 
be  properly  protected  from 
mechanical  injury  and  from 
the  injurious  action  on  its  in- 
sulating qualities  of  gases  or 
liquids  which  may  come  in 


FIG.  280.  —  Duplex    (Twin-conductor)    Electric 
Light  Cable. 


contact  with  it  when  under- 
ground. 

Before    the  lead   covering 

is  put  on  the  insulated  wires,  a  number  of  them  are  usually  "  laid  up  " 
or  bunched  into  a  Cable,  for  telephone  or  telegraph  work;  while  a  single 
wire,  or  at  most  three  or  four  wires, 
are  used  for  power  and  lighting,  and 
the  lead  is  put  around  the  whole. 
Figure  279  shows  a  single  under- 
ground conductor,  and  Figure  280  a 
"  duplex  "  cable  ;  that  is,  one  with 
two  conductors,  and  the  lead  put 
around  both  insulated  conductors. 
The  lead  may  be  put  on  by  pulling 
the  cabled  conductors  into  a  lead 

pipe,  or  by  making  a  pipe  around  the  conductors  by  squeezing  melted 
lead  over  them  by  means  of  a  hydraulic  press.  The  end  of  a  tele- 
graph cable  is  shown  in  Figure  281. 

1  Article  190. 


FIG.  281.  —  Illustration  of  Sixty-conduc- 
tor Telegraph  Cable,  with  Conduc- 
tors exposed  to  View  by  stripping 
off  the  Lead  and  Braiding  at  one 
End.  Reduced  size. 


378 


ELECTRICITY   AND   MAGNETISM 


FIG.  282.  — End  View  of 
Conduit  containing 
Twelve  Ducts. 


327.  Underground  Conduits.  —  Underground  cables  are  not  usually 
buried  directly  in  the  ground,  but  are  placed  in  what  are  known  as 
Electric  Conduits.  These  consist  of  pipes  or 
Ducts  made  of  iron,  terra  cotta  or  vitrified  clay, 
cement,  wood,  and  sometimes  other  materials. 
The  ducts  are  sometimes  laid  singly,  but  they 
are  usually  laid  in  sets  surrounded  by  concrete, 
as  shown  in  Figure  282,  which  is  an  end  view 
of  a  conduit  containing  twelve  ducts.  The 
ducts  are  commonly  circular  in  cross  section 
and  three  or  four  inches  in  diameter,  though 
ducts  of  rectangular  cross  sections  and  of  other 
dimensions  are  often  used. 
In  order  that  cables  may  be  placed  in  the  conduits,  arrangements  for 
getting  at  the  ducts  must  be  made.  This  is  done  by  building  cable 
Manholes  at  intervals  along  the  con- 
duit. These  are  usually  brick  vaults, 
six  or  seven  feet  deep  and  several 
feet  in  diameter,  which  are  covered 
at  the  street  surface  by  cast  iron 
covers.  Sections  of  the  conduit 
terminate  on  opposite  sides  of  the 
manholes,  as  shown  in  Figure  283. 
The  manholes  are  placed  at  inter- 
vals of  about  three  hundred  feet  in 
straight  parts  of  a  conduit  and  also 
at  turns. 

When  a  conduit  with  its  manholes 
is  completed,  the  cables  are  drawn 

into  the  ducts,  One  Section  at  a  time.     FlG-  283-  —  Large  Cable  Manhole,  showing 

The  sections  of  each  cable  must  be 
jointed  together  in  the  manholes. 
To  do  this,  the  conductors  are  first  jointed  in  the  usual  manner,  as 
already  described,1  and  their  joints  are  separately  insulated.  Finally,  a 
short  piece  of  lead  pipe  is  placed  over  the  bunch  of  joints  and  is 

1  Article  324. 


Cable  fed  from  Reel  into  Duct  through 
which  it  is  being  pulled.    DD,  ducts. 


LINE  CONSTRUCTION  379 

soldered  at  both  ends  to  the  lead  covering  by  a  plumber's  "  wiped 
joint."  This  makes  the  joint  moisture  proof  if  the  work  is  properly 
done.  Making  cable  joints  requires  the  greatest  care  to  avoid  the 
entrance  of  moisture  into  the  cable,  and  it  is  always  necessary  to 
handle  open  cable  ends  with  extreme  caution.  The  ends  should 
always  remain  sealed  except  when  work  is  to  be  done  on  them.  , 

When  a  cable  is  to  be  pulled  into  a  duct,  a  strong  rope  must  first  be 
passed  through  the  duct  so  that  it  may  be  used  in  drawing  through  the 
cable.  Several  plans  are  in  use  for  getting  the  rope  through.  A  light 
runner  which  nearly  fills  the  end  of  the  duct  may  be  attached  to  a  cord, 
and  the  runner,  with  the  cord  trailing  after  it,  may  be  sucked  or  blown 
through  the  duct  by  a  mechanical  blower.  The  cord  may  then  serve  to 
draw  the  rope  after  it.  But  the  commonest  plan  for  getting  the  rope 
through  a  duct  is  called  "  rodding  "  the  duct.  A  large  number  of  rods 
made  of  hickory,  bamboo,  or  the  like,  about  a  yard  long,  are  provided 
with  metal  ferrules  and  couplings  at  each  end.  One  of  these  rods  is 
slipped  into  one  end  of  a  duct  by  a  man  standing  in  a  manhole 
and  another  rod  is  then  coupled  to  the  end  of  the  first.  The  second 
rod  is  then  pushed  into  the  duct  (while  it  pushes  the  first  before  it), 
and  a  third  rod  is  coupled  to  the  end  of  the  second.  This  process  is 
continued  until  the  first  rod  is  pushed  through  the  duct  into  the  next 
manhole.  A  rope  is  then  attached  to  the  end  rod,  the  rods  are  with- 
drawn (while  they  are  uncoupled,  one  by  one),  and  the  rope  is  drawn 
into  the  duct  after  the  rods.  When  the  last  rod  is  withdrawn,  the  rope 
lies  extended  from  end  to  end  of  the  duct  and  may  be  used  to  draw  in 
a  cable. 

A  "  leading-in  wire  "  may  be  put  in  the  ducts  when  they  are  laid,  and 
and  it  may  then  be  used  to  draw  in  the  rope ;  but  this  is  not  usually 
considered  desirable  or  convenient. 

A  second  method  of  laying  underground  conductors  for  the  distribu- 
tion of  electric  current  is  often  called  the  solid  or  built-in  system, 
because  the  insulated  conductors  with  their  protecting  conduit  are  laid 
in  the  ground  together.  In  this  case,  if  any  harm  comes  to  either  the 
conductor  or  its  insulation,  the  street  must  be  dug  up  at  the  place  of 
"  trouble  "  before  repairs  can  be  made,  and  for  this  reason  new  plants 
do  not  often  now  install  such  systems.  With  the  "  drawing-in  "  system, 


380  ELECTRICITY  AND   MAGNETISM 

repairs  may  be  made  by  simply  pulling  out  that  section  of  cable  between 
two  manholes  which  contains  the  injury,  and  replacing  it  with  a  piece 
of  good  cable. 

The  "  built-in  "  system  has  been  used  for  low  pressure  distribution 
of  electric  current,  and  for  this  purpose  gives  excellent  satisfaction. 
Nearly  all  the  great  electric  illuminating  companies  in  our  large  cities 
which  use  the  three-wire  system  have  older  conductors  laid  in  this  man- 
ner. For  high  pressure  distribution,  the  "  built-in  "  system  of  under- 
ground conductors  is  not  as  satisfactory  as  the  "  drawing-in  "  system. 

The  most  commonly  used  arrangement  of  the  "  built-in  "  system  is  that 
known  as  "  Edison  tubing."  This  was  introduced  nearly  a  score  of  years 
ago,  and  was  used  in  its  original  form  in  the  laying  of  the  conductors 
connected  with  the  old  Pearl  Street  Central  Station  in  New  York  City, 
the  first  great  central  station  for  the  general  distribution  of  the  electric 
current.  Edison  tubing  was  the  earliest,  and  for  many  years  the  only 
scheme,  in  which  the  details  of  a  general  underground  system  for  dis- 
tributing electric  current  were  satisfactorily  worked  out. 

On  account  of  the  experience  gained  in  laying  the  conductors  for  the 
various  large  Edison  electric  illuminating  companies,  the  system  of  tub- 
ing has  been  considerably  changed  since  its  first  introduction.  As  the 
tubes  are  now  made,  they  usually  contain  three  copper  rods  —  the  posi- 
tive, negative,  and  neutral  conductors  of  the  three-wire  system.  These 
rods,  which  are  somewhat  over  twenty  feet  long,  are  each  wound  with  a 
spiral  of  manila  rope,  and  are  then  laid  side  by  side 
but  separated  from  each  other  by  the  ropes.  An- 
other spiral  of  rope  is  wound  around  the  bunch  to 
hold  the  conductors  firmly  together.  The  bunch 
of  three  conductors  is  placed  in  an  iron  pipe  twenty 
feet  long,  in  such  a  way  that  the  copper  rods  stick 
out  a  few  inches  at  each  end.  One  end  of  the  pipe 
FIG.  284.  —  Sectional  or  tube  is  then  connected  to  a  pump  by  means  of 
View  of  Edison  Tube  which  vacuum  is  created  in  the  tube,  and,  finally, 

with    Three    Equal 

Conductors.  hot,  black  insulating  compound  is  pumped  into  the 

tube  until  all  the  open  space  inside  of  it  is  filled. 

The  insulating  compound  is  of  a  bituminous  nature  and  hardens  when  it 

is  permitted  to  cool.     Figure  284  shows  a  cross  section  of  a  "tube  "  in 


LINE  CONSTRUCTION 


381 


which  A,  A,  A  are  the  copper  conductors,  C  is  the  iron  pipe,  and  B  is 
the  insulating  compound.  Figure  285  shows  a  length  of  the  completed 
tubing,  showing  the  form  in  which  it  is  delivered  from  the  factory  to  be 
laid  in  the  ground.  For  the  purpose  of  laying  the  tubes,  a  trench  is 
dug,  and  the  twenty-feet  lengths  are  laid  down  end  to  end.  The  con- 


FiG.  285.  —  Illustration  of  Edison  Tube. 

ductors  in  successive  tubes  are  joined  by  means  of  flexible  copper  con- 
nectors (Fig.  286)  having  solid  copper  heads  with  holes  which  slip  over 
the  ends  of  the  rods  where  they  are  soldered  fast.  Ball-like  caps  are 
bolted  fast  to  the  tube  ends,  and  over  these  is  bolted  a  split  coupling  box 
which  covers  the  joint.  In  the  top  of  this  coupling  box  is  a  hole 
through  which  hot  insulating  com- 
pound may  be  poured  when  the 
joint  is  completed,  and  the  hole 
is  then  covered  with  an  iron  cap. 

The  arrangement  here  described 
is  very  satisfactory,  since  it  offers 
an  electric  company  the  same  ease 
as  a  gas  company  or  a  water  com- 
pany in  making  connections  to 
houses.  A  branch  to  a  house,  or  Service  Connection,  as  it  is  called, 
may  be  connected  to  the  main  conductors  at  any  coupling  box  by 
simply  changing  the  plain  box  to  a  T-shaped  box  and  running  a  branch 
into  the  house.  With  "  drawn-in  "  systems,  access  to  the  conductors 
can  easily  be  obtained  only  at  the  manholes,  and  house-to-house  dis- 
tribution cannot  be  so  conveniently  made. 

Several  different  arrangements  of  "  built-in "  conductors  have  been 
used  in  England,  France,  and  Germany.  One  of  these  consists  of  a 
simple  brick,  concrete,  or  cast-iron  trench,  or  culvert,  in  which  the 
copper  rods  or  bars  used  for  conductors  are  placed  on  porcelain  insula- 
tors. One  of  the  most  remarkable  arrangements  of  the  "  built-in  " 
system  is  that  constructed  some  years  ago  in  London  to  conduct  electric 


FIG.  286.  —  Coupling  Box  for  Edison  Tubes, 
showing  Flexible  Connectors. 


382 


ELECTRICITY  AND    MAGNETISM 


current  by  the  two-wire  system  from  the  noted  Deptford  Central  Station 
into  the  heart  of  the  city.  The  conductors  in  this  case  are  enclosed  in 
an  iron  pipe,  as  are  the  conductors  in  the  Edison  system,  but  the  con- 
ductors themselves  are  copper  tubes  placed  one  inside  of  the  other 
instead  of  being  rods  placed  side  by  side.  The  space  between  the  con- 
ductors is  filled  with  insulation,  which  consists  of  brown  paper  soaked  in 
an  insulating  compound.  The  same  kind  of  insulation  is  also  placed 
between  the  outer  conductor  and  the  iron  protecting  pipe.  This  con- 
ducting system  was  designed  and  laid  down  to  transmit  current  at  the 
then  enormous  and  unusual  pressure  of  10,000  volts,  and  it  served  its  pur- 
pose very  well.  As  the  tube  could  not  be  made  in  lengths  much  greater 
than  twenty  feet,  jointing  the  lengths  together  was  a  matter  of  much 
difficulty  on  account  of  the  concentric  arrangement  of  the  conductors. 

328.  Arrangement  of  Distributing  Systems.  —  In  order  that  the 
electrical  pressure  may  be  kept  the  same  at  all  points  on  a  system  of 
electric  or  power  conductors  which  cover  a  large  district,  the  conduct- 
ors must  be  divided  into  Feeders  and  Mains.  The  mains  consist  of  the 
conductors  to  which  lamps  or  motors  are  directly  connected.  These 
are  carried  all  through  the  streets  of  the  district  in  which  current  is  to 
be  supplied,  and  they  are  often  joined  into  a  network  by  means  of  fuses 
located  in  manholes  or  Junction  Boxes  at  street  corners.  The  current 

is  supplied  to  the  mains  at  cer- 
tain central  points  called  Feeding 
Points  by  means  of  feeders  which 
run  directly  to  the  feeding  points 
from  the  central  station  where  the 
current  is  generated.  Figure  287 
is  a  diagram  representing  the  ar- 


FIG.  287.-  illustration  of  tie  Arrangement  rangement  of  feeders  and  mains, 
of  the  Feeders  and  Mains  in  the  Con-  The  points  marked  I,  2,  3,  4,  are 
ducting  Network  of  an  Electric  Light  the  feeding  points>  and  ^  ^  C> 

D,  are  houses  to  which  current  is 

supplied  through  service  connections.  The  figure  shows  three  wires 
in  each  main  and  feeder,  as  is  required  in  a  three-wire  system.  By 
carefully  calculating  the  resistance  of  each  main  and  feeder  before  the 
system  is  constructed  it  is  possible  to  get  a  very  uniform  pressure  over 


LJLmJ 


ELECTRICAL  DISTRIBUTION  OF   POWER  383 

the  whole  distributing  system.  In  order  that  the  dynamo  men  may  reg- 
ulate the  pressure  of  the  dynamos  in  the  central  station  so  as  to  keep  the 
pressure  uniform  at  the  feeding  points,  it  is  necessary  to  have  voltmeters, 
or  pressure  indicators,  in  the  dynamo  room,  which  show  the  pressure  at 
the  feeding  points.  For  this  purpose  wires  called  Pressure  Wires  are  run 
from  the  feeding  points  to  the  voltmeters  in  the  dynamo  room.  A  some- 
what similar  network  of  pipes  is  sometimes  used  in  the  distribution  of 
gas  and  water  in  large  cities. 

329.  Determination  of  Wire  Sizes.  —  The  sizes  of  the  wire  used  in 
electric  light  and  power  systems  are  usually  determined  with  a  view  to 
preventing  an  excessive  Drop  in  pressure,  caused  by  the  current  which 
flows  along  it,1  with  its  corresponding  loss  of  energy  in  the  conductors. 
Copper  wire  is  ordinarily  used,  and  the  resistance  of  a  commercial  wire 
which  has  a  cross  section  of  one  circular  mil  and  a  length  of  one  foot 
(that  is,  one  mil-foot)  is  about  10.5  ohms  at  ordinary  temperatures.2 
The  resistance  of  any  copper  wire  is,  therefore,  equal  to  10.3  times  its 
length  in  feet  divided  by  its  cross  section  in  circular  mils,  since  the  resist- 
ances of  conductors  are  proportional  directly  to  their  lengths  and 
inversely  to  their  cross  sections.3 

As  a  formula  which  is  easy  to  remember,  the  statement  in  italics  may 

be  written 

10.5^ 

CM. 

Now  the  drop  in  pressure  in  a  line  of  resistance  R,  when  current  C 
flows  through  it,  is 5 

y=  R  x  c, 

and  therefore, 

v_  10.5  x  L  x  C 
CM. 

from  which  it  is  at  once  to  be  seen  that 

CM.  =  I0'5  X  Z  x  C 

1  Articles  107  and  261.  2  Article  100.  3  Article  101. 

4  Tables  giving  the  sizes  of  wires  for  electric  lighting  nearly  always  have  a  column 
which  shows  the  cross  sections  in  circular  mils.  5  Article  92. 


384  ELECTRICITY  AND   MAGNETISM 

The  last  formula  shows  that  the  wire  which  conveys  the  current  from  a 
dynamo  to  lamps  or  motors  must  have  the  number  of  Circular  Mils  in  its 
cross  section  which  is  equal  to  the  specific  resistance  (for  copper  10.5) 
times  the  total  Length  of  wire  infect  multiplied  by  the  amperes  of  Current 
transmitted,  and  divided  by  the  Volts  lost  in  the  line. 

Suppose,  for  instance,  we  wish  to  know  how  large  a  copper  wire  we 
must  use  to  transmit  50  amperes  over  a  wire  500  feet  long  with  a  drop 
of  10  volts  in  pressure.  Putting  50  in  place  of  C,  500  in  place  of  Z,  and 
10  in  place  of  F,  we  have, 

10.5  x  500  x  50 
C.M.  =  —  —  =  26,250. 

Or,  a  wire  having  a  cross  section  of  26,250  circular  mils  must  be 
used.  If  a  larger  wire  is  used,  there  will  be  less  than  10  volts  lost  in  the 
line  ;  and  if  a  smaller  wire  is  used,  the  loss  will  be  greater.  Referring  to 
the  table  given  in  the  next  article,  it  will  be  found  that  a  No.  6  wire 
drawn  to  the  B.  &  S.  gauge  is  of  very  nearly  the  size  required. 

Suppose  now  it  is  desired  to  supply  current  to  an  electric  arc  lamp 
which  takes  5  amperes  and  requires  a  pressure  of  100  volts  for  its  opera- 
tion. Suppose  this  lamp  is  1000  feet  from  the  electric  generating  appa- 
ratus, which  means  that  the  length  of  wire  for  the  complete  circuit  will 
be  2000  feet.  Also  suppose  that  the  generating  apparatus  will  supply 
105  volts  pressure  ;  then  in  order  to  have  100  volts  at  the  lamp  there 
must  be  only  5  volts  lost  in  the  line.  Using  the  formula  as  before,  we 
have, 

„  ...      10.5  x  2000  x  5 

CM.  =  -  —  —  2 1,000, 

or  about  a  No.  7  B.  &  S.  wire. 

The  circuit  arrangement  in  the  last  example  is  quite  analogous  to  the 
hydraulic  illustration  given  in  Article  102,  Figure  46,  where  there  is  only 
a  small  loss  of  water  pressure  in  the  large  connecting  tank  and  most  of 
the  pressure  is  lost  in  the  small  pipes.  In  the  case  we  are  considering, 
most  of  the  electric  pressure  is  absorbed  in  the  electric  arc  and  the  resist- 
ance of  the  lamp,  only  about  5  per  cent  of  it  being  used  in  sending  the 
current  through  the  connecting  wires. 

If  a  group  of,  say,  five  similar  lamps  had  been  connected  in  parallel  to 


ELECTRICAL  DISTRIBUTION  OF   POWER 


385 


the  ends  of  the  wire,  there  would  have  been  25  amperes  to  transmit. 
This  is  much  as  if  there  were  five  equal  water-wheels  taking  water  from 
one  dam.  All  the  wheels  together  would  take  five  times  as  much  cur- 
rent as  one  wheel,  and  each  would  utilize  the  full  water  pressure  of  the 
dam,  as  is  described  in  the  illustration  in  Article  259. 

330.  Properties  of  Copper  Wire.  —  The  following  table  gives  useful 
data  relating  to  the  dimensions  and  properties  of  copper  wire.  The 
first  column  gives  the  numbers  by  which  different  sizes  of  wire  are 
designated  in  the  B.  &  S.  gauge,  which  is  the  gauge  most  largely  used 
in  this  country.  The  second  column  gives  the  diameters  of  the  respec- 
tive wires.  The  third  column  gives  the  circular  mils1  in  each  cross  sec- 
tion ;  the  fourth,  the  weights  of  bare  copper  wire  per  thousand  feet. 
And  the  fifth  and  sixth  columns  give  the  resistances  of  a  thousand  feet 
of  each  size  of  copper  wire  at  the  temperatures  of  60°  and  75°  Fahren- 
heit, respectively. 

CHARACTERISTICS  OF  COPPER  WIRE  WHICH  is  DRAWN  TO  THE  BROWN  AND  SHARP 

(B.  &  S.)  GAUGE 


B.  &  S. 
GAUGE 

DIAMETER 
IN  MILS 

AREAS  IN 
CIRCULAR  MILS 

WEIGHT  PER 
1000  FEET 

RESISTANCE  PER  1000  FT.,  IN  OHMS 

60°  F. 

75°  F. 

oooo 

460 

211,600 

641 

.048    II 

.049  66 

000 

410 

l68,IOO 

5°9 

.060   56 

.062  51 

oo 

365 

133.225 

403 

.076  42 

.078  87 

0 

325 

105,625 

320 

.096  39 

.099  48 

I 

289 

83,521 

253 

.121  9 

.125  8 

2 

258 

66,564 

202 

.152  9 

•157  9 

3 

229 

52,441 

159 

.194  i 

.200  4 

4 

204 

41,616 

126 

.244  6 

•252  5 

5 

182 

33.124 

100 

•3°7  4 

.317    2 

6 

162 

26,244 

79 

•387  9 

.400  4 

7 

I44 

20,736 

63 

.491     . 

.506  7 

8 

128 

16,384 

50 

.621  4 

.641  3 

9 

II4 

12,996 

39 

-.783  4 

.808  5 

10 

IO2 

10,404 

32 

•978  5 

I.OI 

1  1 

91 

8,28l 

25 

1.229 

1.269 

1  Article  99. 


386 


ELECTRICITY  AND   MAGNETISM 


CHARACTERISTICS  OF  COPPER  WIRE  WHICH  is  DRAWN  TO  THE  BROWN  AND  SHARP 
(B.  &  S.)  GAUGE  {Continued} 


B.  &S. 
GAUGE 

DIAMETER 
IN  MILS 

AREAS  IN 
CIRCULAR  MILS 

WEIGHT  PER 
1000  FEET 

RESISTANCE  PER  1000  FT.,  IN  OHMS 

60°  F. 

75°  F. 

12 

81 

6,561 

20 

1-552 

1.601 

13 

72 

5,184 

*S-7 

1.964 

2.027 

14 

64 

4,096 

12.4 

2.485 

2-565 

15 

57 

3,249 

9.8 

3-133 

3.234 

16 

5i 

2,  60  1 

7-9 

3.9I4 

4.04 

J7 

45 

2,025 

6.1 

5.028 

5.189 

18 

40 

1,  6OO 

4.8 

6.363 

6.567 

19 

36 

1,296 

3-9 

7.855 

8.108 

20 

32 

1,024 

3-i 

9.942 

10.26 

21 

28.5 

812.3 

2-5 

12-53 

12.94 

22 

25-3 

640.1 

1.9 

15.9 

16.41 

23 

22.6 

510.8 

!-5 

19.93 

20.57 

24 

20.1 

404 

1.2 

25.2 

26.01 

25 

17.9 

320.4 

•97 

31-77 

32.79 

26 

15-9 

252.8 

•77 

40.27 

41.56 

27 

I4.2 

201.6 

.61 

5°49 

52.11 

28 

12.6 

158.8 

.48 

64.13 

66.18 

29 

"•3 

127.7 

•39 

79-73 

82.29 

30 

10.0 

100 

•3 

101.8 

105.1 

31 

8.9 

79.2 

.24 

128.5 

132.7 

32 

8 

64 

.19 

I59-I 

164.2 

33 

7-1 

5°4 

•'5 

202 

208.4 

34 

6-3 

39-7 

.12 

256.5 

264.7 

'    35 

5.6 

314 

•095 

324.6 

335-1 

36 

5 

25 

.076 

407.2 

420.3 

An  inspection  of  this  table  shows  that  with  every  tabular  difference  of 
three  "numbers  "  in  the  sizes  of  the  wires  there  exists  a  ratio  of  nearly 
2  to  i  in  the  areas  of  the  wires.  Thus  the  cross  section  of  No.  18  has 
an  area  of  1600  circular  mils,  while  No.  15  has  an  area  of  3249  circular 
mils,  and  No.  21  has  an  area  of  812.3  circular  mils.  Again,  the  cross 
section  of  No.  o  has  an  area  of  105,625  circular  mils,  while  No.  oooo 
has  an  area  of  211,600  circular  mils,  and  No.  3  has  an  area  of  52,441 
circular  mils. 


ELECTRICAL   DISTRIBUTION   OF   POWER 


387 


The  Birmingham  or  Stubs  wire  gauge  (B.  W.  G.)  is  sometimes  used  to 
designate  copper  wire,  and  quite  commonly  used  to  designate  iron  wire. 
The  diameters  of  wires  drawn  to  this  gauge  are  given  in  the  following 
table  :  — 


NUMBER 

DIAMETER 
IN  MILS 

NUMBER 

DIAMETER 

IN  MILS 

NUMBER 

DIAMETER 

IN  MILS 

NUMBER 

DIAMETER 
IN  MILS 

oooo 

454 

7 

1  80 

17 

58 

27 

16 

000 

425 

8 

I65 

18 

49 

28 

14 

oo 

380 

9 

148 

19 

42 

29 

13 

o 

340 

10 

134 

20 

35 

3° 

12 

I 

300 

ii 

120 

21 

32 

31 

10 

2 

284 

12 

109 

22 

28 

32 

9 

3 

259 

ij 

95 

23 

25 

33 

8 

4 

238 

H 

83 

24 

22 

34 

7 

5 

220 

15 

72 

25 

2O 

35 

5 

6 

203 

16 

65 

26 

18 

36 

4 

It  will  be  noticed  that  the  diameters  given  in  this  table  through  a 
limited  range  (from  No.  5  to  No.  ij)  do  not  greatly  differ  from  the 
diameters  of  the  wires  in  the  B.  &  S.  table  through  a  similar  range, 
which  begins  at  No.  j  and  ends  at  No.  ij. 

331.  The  Electrical  Distribution  of  Power.  —  The  volts  "  drop  " l  in  a 
transmission  wire  is  fixed  by  the  pressure  at  the  dynamo  or  battery,  and 
the  percentage  of  that  pressure  which  may  be  reasonably  sacrificed  in 
the  transmission.  For  instance,  if  the  pressure  at  the  dynamo  is  125 
volts,  and  it  is  considered  reasonable  to  allow  a  loss  of  pressure  equal  to 
10  per  cent  of  this,  then  the  "  drop  "  of  pressure  is  12.5  volts. 

It  has  not  been  found  commercially  possible  to  produce  incandescent 
lamps  for  a  higher  pressure  than  about  115  volts  until  recently,  and, 
consequently,  nearly  all  incandescent  lighting  with  continuous  currents 
is  done  at  a  pressure  between  100  volts  and  115  volts,  and  each  sixteen- 
candle-power  lamp  takes  about  one-half  an  ampere  of  current.  If  by 
some  means  the  pressure  at  the  lamps  could  be  doubled  without  any 
change  in  the  amount  of  light  given  out  for  each  hundred  watts,  then 


1  Article  329. 


388  ELECTRICITY   AND    MAGNETISM 

each  sixteen-candle-power  lamp  would  require  only  about  one-fourth  of 
an  ampere. 

The  pressure  being  doubled,  the  number  of  volts  in  a  given  percentage 
loss  would  also  be  doubled.  We  see,  therefore,  that  the  current  divided 
by  the  "  drop  "  is  only  one-fourth  as  great  with  the  double  pressure,  so 
that  the  wires  required  to  carry  current  a  fixed  distance  for  2OO-volt 
lamps  need  be  only  one-fourth  as  heavy  as  those  required  to  carry  current 
for  the  same  number  of  loo-volt  lamps.  Or,  putting  the  statement  in 
another  way,  the  weight  of  copper  which  is  required  to  supply  current 
at  a  fixed  percentage  loss  of  pressure  to  a  number  of  loo-volt  lamps  at 
a  certain  distance  from  the  dynamo  will,  at  double  the  pressure,  serve  to 
supply  four  times  as  many  lamps. 

In  the  same  way,  it  may  be  seen  that  if  the  pr.essure  is  increased  from 
100  volts  to  300  volts,  the  wires  required  to  convey  a  given  supply  of 
power  a  certain  distance  may  be  reduced  to  one-ninth  as  great  a  cross 
section,  and,  therefore,  to  one-ninth  the  weight  of  those  required  for  the 
loo-volt  distribution. 

The  general  rule  may  be  given  as  follows  :  When  a  given  amount  of 
power  is  transmitted  by  electricity  over  a  certain  distance  at  a  fixed  per- 
centage loss,  the  cross  section  of  the  wires,  and,  therefore,  their  iv eight,  is 
in  inverse  proportion  to  the  square  of  the  pressure. 

This  rule  applies  equally,  whether  the  current  is  used  for  producing 
light,  operating  stationary  motors,  running  street  cars,  or  for  other  pur- 
poses. The  distribution  of  electricity  by  means  of  wires  from  a  dynamo 
at  one  point  to  be  used  'at  other  points  is  Electrical  Distribution  of 
Power,  whatever  may  be  the  purposes  for  which  the  current  is  used,  and 
the  laws  of  transmission  and  distribution  apply  equally  in  one  case  as 
another.  Electric  lamps,  arc  or  incandescent,  may  be  operated  on  the 
same  circuits  with  electric  motors  or  electric  heaters ;  or  electric  lamps 
may  be  taken  out  of  a  circuit  and  electric  motors  put  in  their  place,  or 
vice  versa,  without  altering  the  conditions.  It  is  well  known  that  in 
many  cities  electric  arc  and  incandescent  lamps,  stationary  motors,  and 
electrically  heated  flat-irons  and  curling  irons  are  all  furnished  with  the 
power  necessary  for  their  operation  from  the  same  circuits.  Electric 
street  cars  are  often  furnished  with  light,  heat,  and  power  from  the 
current  conveyed  to  the  car  by  the  trolley  wire. 


ELECTRICAL   DISTRIBUTION   OF   POWER  389 

I 

QUESTIONS 

1.  What  wood  is  ordinarily  used  for  electric  poles  ?    For  cross  arms  ?    For  pins  ? 

2.  Why  are  cross  arms  set  in  pairs  facing  each  other  ? 

3.  How  should  a  pole  line  be  guyed  or  braced  ? 

4.  What  is  the  smallest  size  of  copper  wire  that  should  be  used  on  a  pole  line  ? 

5.  What  size  of  wire  is  used  in  arc  circuits  ? 

6.  Do    overhead  telegraph   and   telephone  wires  ordinarily  have  an  insulated 
covering  ? 

7.  Why  are  electric  light  circuits  not  grounded  ? 

8.  How  is  a  pole  line  erected  ? 

9.  How  are  wire  joints  made  ? 

10.  What  happens  if  the  insulation  of  a  telegraph  line  is  poor  ? 

11.  Should  electric  light  lines  be  well  insulated  ? 

12.  What  is  weather-proof  wire  ? 

13.  What  kind  of  insulating  supports  are  used  for  supporting  overhead  telephone 
and  telegraph  wires  ?     For  light  and  power  wires  ? 

14.  Why  are  lead  coverings  put  on  underground  cables  ? 

15.  What  are  electric  conduits  ?     Manholes  ? 

1 6.  How  are  light  and  power  cables  insulated  ? 

17.  Why  is  crinkled  paper  used  in  insulating  the  wires  of  telephone  cables  ? 

18.  Compare  telegraph,  telephone,  and  power  cables. 

19.  How  are  lead-covered  cable  joints  made  ? 

20.  What  are  "  built-in  "  underground  systems  ? 

21.  Describe  the  Edison  "built-in"  system. 

22.  What  are  feeders  ?     What  are  mains  ? 

23.  Why  are  the  conductors  of  a  constant  pressure  lighting  system  divided  into 
feeders  and  mains  ? 

24.  What  is  the  formula  for  calculating  the  sizes  of  wires  ?     How  is  it  obtained? 

25.  What  is  the  cross  section  (in  circular  mils)  of  a  No.  oooo  wire  drawn  to  the 
B.  and  S.  gauge  ?     Of  a  No.  12  wire  ? 

26.  What  is  meant  by  "  volts  drop  "  ? 

27.  How  is  the  "  volts  drop  "  in  a  wire  determined  ? 

28.  How  much  larger  a  wire  is  required  to  supply  current  to  a  group  of  loo-volt 
lamps  than  to  supply  it,  with  the  same  loss  of  pressure,  to  a  group  of  2OO-volt  lamps 
which  absorbs  the  same  amount  of  power  ? 

29.  For  transmitting  a  given  amount  of  power,  how  does  the  cross  section  of  the 
wires  depend  upon  the  pressure  ? 

30.  For  transmitting  a  given  amount  of  power  at  a  fixed  pressure,  how  does  the 
cross  section  of  the  wires  depend  on  the  distance  of  transmission  ? 

31.  How  does  the  weight  of  conductors  depend  on  the  pressure  when  the  "drop  " 
is  a  fixed  percentage  of  the  pressure  ?     How  does  the  weight  depend  on  the  distance 
of  transmission  when  the  pressure  is  fixed  ? 


390 


ELECTRICITY  AND   MAGNETISM 


332.    Series  and  Parallel  Systems.  —  The  series  system  of  electric 
lighting  in  which  all  the  lamps  are  in  series  1  is  illustrated  in  Figure  288. 

Suppose  that  the  dynamo 
in  this  figure  generates  550 
volts,  and  each  of  the  10 
lamps  calls  for  50  volts; 
then  the  lamps  will  use  a 
total  of  500  volts,  leaving 
9  per  cent.  The  line  must 


1AMO^~} 


FIG.  288.  —  Diagrammatic  Illustration  of  Series  Ar- 
rangement of  Lamps  and  Dynamo.  L,  L,  L, 
Lamps. 


50  volts  for  the  loss  in  the  line,  or  about 
be  designed  accordingly. 

Figure  289  indicates  the  parallel  system  of  electric  lighting  in  its  sim- 
plest form.  If  in  this  case  the  lamps  call  for  100  volts  and  the  dynamo 
generates  in  volts,  there  A 

will  be  10  per  cent,  or  n     DYNA 
volts,    to    be    lost    in    the 

line.        In    calculating     the     FIG.  289.  — Diagrammatic  Illustration  of  Parallel  A r- 


rangement  of  Lamps  and   Dynamo.      L,  L,  L, 
Lamps. 


size  of  wire   the   distance 

taken  would  be  from  the 

dynamo  to  AA,  the  most  central  point  in  the  group  of  lamps.     If  the 

distribution  is  more  complex,  as  in  Figure  290,  and  a  total  drop 

of,  say,  10  per  cent  is  to  be 
allowed  from  dynamo  to  lamps, 
the  main  line  or  feeder  would 
be  calculated  from  the  dynamos 
to  AA  (the  central  point  from 
which  the  branches  or  mains 
leave  the  feeders)  with  a  drop 
of,  say,  8  per  cent.  Then  each 
branch  would  be  calculated  as 
in  the  simple  case  illustrated 
in  Figure  289,  using  the  re- 
maining drop,  which  in  this 
case  is  2  per  cent.  The  point 

AA  in  the  figure  is  called  the  Centre  of  Distribution  of  the  system  of 

conductors. 


-o- 
-o- 

FlG.  290.  —  Diagrammatic  Illustration  of  Par- 
allel System  with  Lamps  connected  to  Sev- 
eral Branch  Lines. 


1  Article  252. 


ELECTRICAL  DISTRIBUTION   OF   POWER  39 1 

333.  Multiple  Series  Systems.  —  It  is  very  easy  to  make  the  pressure 
quite  high  in  circuits  which  are  arranged  to  transmit  power  from  a  cen- 
tral station  to  electric  motors  alone,  and  thus  keep  the  weight  of  the  wires 
required  within  a  reasonable  limit,  since  the  electric  motors  may  have 
their  windings  designed  for  any  reasonable  pressure.     Five  hundred  volts 
is  the  pressure  quite  commonly  used  for  direct  current  circuits  which  are 
specially  intended  to  supply^  current 

to  stationary  motors  and  street-car 

motors.      Incandescent  lamps  may 

be  used  on  such  circuits,  but  it  is 

necessary  to  use   them  in  sets  of 

five    loo-volt    lamps    connected   in  1—OOOOO-* 

series    (Fig.    291).      This  is   the   ar-      FIG.  291. -Diagrammatic  Illustration  of 

rangement  which    is    used   for   light-  a    Multiple    Series    Arrangement    of 

ing  electric  cars.     For  general  pur-  TElectric  LamPs  in  which  Sets  of  Jive 

Lamps   in    Series   are   connected   in 
poses,  such  an  arrangement  is  not  Parallel  with  each  other. 

at  all  satisfactory,  because  all  the 

lights  of  each  set  must  either  burn  or  be  extinguished.  It  is  not  pos- 
sible to  have  only  one  or  two  lamps  of  a  set  burn  at  once.  Systems 
of  this  class  are  called  Multiple  Series  Systems. 

334.  Three-wire  System.  —  Two  loo-volt  lamps  put  in  series  on  a 
2oo-volt  circuit  are  more  satisfactory  to  use  than  five  lamps  in  a  set,  but 
even  such  an  arrangement  is  not  suitable  for  general  service.     But  the 
Three-wire  System  effects  nearly  as  much  saving  in  copper  as  is  brought 
about  by  doubling  the  pressure  through  arranging  the  lamps  in  series  of 
two,  and  yet  the  individual  lamps  of  the  three-wire  system  are  entirely 
independent.     A  diagram  of  the  arrangement  of  the  three-wire  system 
is  shown  in  Figure  292.    A  and  B  are  two  dynamos  ;  the  positive  termi- 
nal of  the  first  is  connected  to  the  positive  line  wire,  the  negative  termi- 
nal of  the  first  is  connected  to  the  positive  terminal  of  the  second,  and 
the  negative  terminal  of  the  second  is  connected  to  the  negative  line  wire. 
A  third  wire  called  the  Neutral  Wire  is  connected  at  a  point  between  the 
two  dynamos  and  runs  out  along  the  line  with  the  positive  and  negative 
wires.     Some  of  the  electric  lamps  are  connected  in  parallel  between  the 
positive  and  neutral  wires,  and  the  others  are  connected  in  parallel  be- 
tween the  negative  and  neutral  wires,  the  lamps  being  arranged  so  that 


392 


ELECTRICITY  AND   MAGNETISM 


-WIRE 


FlG.  292.  —  Diagrammatic  Illustration  of  Balanced 
Three-wire  System  of  Electrical  Distribution. 


the  numbers  on  the  two  sides  of  the  system  are  as  nearly  equal  as  possible, 
and   so   that   the   numbers  of   lamps  likely  to  burn  at  one  time   are 

nearly  equal  for  the  two  sides. 
When  this  condition  is  ful- 
filled as  shown  in  the  figure, 
the  system  is  said  to  be  Bal- 
anced, and  the  current  flows 
from  the  positive  pole  of  the 
first  dynamo  through  the  posi- 
tive wire  to  the  lamps  on  the 
positive  side,  through  these 

lamps  to  the  neutral  wire,  and  thence  directly  through  lamps  on  the 
negative  side  to  the  negative  wire,  and  finally  through  the  negative  wire 
to  the  negative  terminal 
of  the  second  dynamo. 

If  the  system  is  bal- 
anced, no  current  returns 
to  the  dynamos  through 
the  neutral  wire,  and  the 
dynamos  operate  exactly 
as  though  they  were  sim- 
ply connected  in  series. 
The  function  of  the  neu- 
tral wire  is  then  to  dis- 
tribute the  current  from 
the  lamps  on  the  positive  side  of  the  system  to  those  on  the  negative 

side.  But  if  the  system  is  not  bal- 
anced and  more  lamps  are  in  use 
H4  on  one  side  of  the  system  than 
on  the  other,  the  extra  current  is 
delivered  by  or  returned  to  the 
dynamos  through  the  neutral  wire. 

FIG.  294. -Diagrammatic  Illustration  of  Un-  Fl'gUre    293    shows    by  the    arrows 

balanced  Three-wire  System  of  Electrical  the    way    in    which    the    current   IS 

Distribution      The  numerals  indicate  the  distributed   to  the  ]amps  on  a  bal. 
proportional  currents  flowing  in  different 


-o 

-O 

-0 

-o 

0 

-o 

Xs| 

o 

IB         af 

I3    4 

0 

ii 

J 

5» 

2> 

T         | 

+  WIRE 

NEUTRAL 
V.'IRE 


"*~5~ 

'  •*  — 

—  WIRE 

M 

I'M 

-0 

l< 

-O- 

-o 

-o- 

O 

-o- 

~o- 

-o- 

FlG.    293.  —  Diagrammatic    Illustration    of  Balanced 
Three-wire  System  of  Electrical  Distribution. 


parts  of  the  circuit. 


anced  system  in  which  the  lamps 


ELECTRICAL   DISTRIBUTION   OF   POWER  f        393 

are  not  exactly  opposite  each  other.  Figure  294  is  a  diagram  of  a 
three-wire  system  with  more  lights  connected  to  the  positive  than 
to  the  negative  side  of  the  system.  The  arrows  show  the  directions 
in  which  the  current  flows  in  the  wires.  The  positive  wire  carries 
enough  current  to  supply  the  lamps  on  the  positive  side  of  the  system, 
and  the  difference  between  the  current  required  to  supply  the  lamps  on 
the  two  sides  returns  through  the  neutral  wire.  The  positive  dynamo, 
therefore,  carries  more  load  than 
the  negative  dynamo.  If  each 
lamp  is  assumed  to  require  one 
ampere  in  its  operation,  then  the 
numerals  in  the  figure  represent 
the  relative  amounts  of  current 
which  flow  in  the  various  paths 
of  the  circuit. 


In  Figure  295  the  dynamos  of     FlG-  295.  — Diagrammatic  Illustration  of  an 
,  ,.          f.  Hydraulic  Analogy  to  the  Three-wire  Sys- 

the    preceding    figures    are    sup-          tern  of  Electrical  Distribution. 

posed  to  be  replaced  by  pumps 

in  an  analogous  arrangement,  and  the  lamps  are  supposed  to  be  replaced 

by  water  motors.     The  arrows  show  the  directions  of  the  streams  of 

water  in  the  system  of  piping  when  the  pipes  and  motors  are  working. 
The  three-wire  system  is  used  to  a  very  large  extent  by  the  electric 

lighting  companies  all  over  the  world. 

335.    Five- wire  System.  —  Plans  for  increasing  the  dynamo  pressure 

used  to  supply  incandescent  lamps,  such  as  the  three-wire  system  which 
consists  essentially  in  connecting  the  lamps 
practically  in  series  and  yet  making  them 
really  independent  of  each  other  by  means 
of  a  neutral  wire,  may  be  extended.  Figure 
296  is  a  diagram  of  an  arrangement  with  four 
lamps  in  series  and  five  wires.  A,  B,  C,  and 
D  represent  the  dynamos.  This  arrangement, 
known  as  the  five-wire  system,  has  not  come 

FIG.  296.  —  Diagrammatic  II-     into  much  use  in  the  United  States. 

lustration  of  a  Five-wire         33^    Weights  of  Wire  in  Several  Systems. 

System  of  Electrical  Dis- 
tribution. The  weight  of  wire  in  a  three-wire  system  of  a 


394  ELECTRICITY   AND   MAGNETISM 

certain  pressure  at  the  individual  lamps  amounts  to  a  little  more  than 
one-fourth  of  the  weight  required  for  a  two- wire  system  with  the  same 
pressure  at  the  lamps.  Since  the  pressure  from  positive  to  negative- 
wire  in  the  three-wire  system  is  twice  that  of  the  two-wire  system, 
one  might  suppose  that  only  one- quarter  as  great  a  weight  of  wire 
would  be  necessary  in  the  three-wire  system  for  the  distribution  of 
current  for  a  given  number  of  lamps ;  but  this  is  not  true,  because  of 
the  introduction  of  the  neutral  wire  which  adds  to  the  total  weight 
of  wire  required  by  the  three-wire  system.  The  actual  weight  required 
in  the  three-wire  system  is  about  three-eighths  of  that  in  a  two-wire 
system.  This  saving  in  the  weight  of  copper  is  a  very  important 
factor  to  large  electric  lighting  companies,  as  their  copper  feeders  and 
mains  cost  a  great  deal  of  money. 

The  saving  effected  by  the  five-wire  system  is  proportionally  greater 
than  that  brought  about  by  the  three-wire  system,  but  it  causes  greater 
difficulty  in  keeping  the  pressure  perfectly  constant  at  the  lamps. 

337.  The  Alternating  Current  System.  — The  systems  of  distribution 
heretofore  described  are  suitable  for  both  direct  and  alternating  currents. 
The  possibility  of  transforming  the  pressure  of  alternating  currents,  how- 
ever, permits  the  use  of  a  high  pressure  on  the  transmission  lines  with- 
out increasing  the  pressure  on  the  lamp  circuits.    Figure  297  represents 
such  a  system  where  the  feeders  carry  from  1000  to  2000  volts  press- 
ure  and   the   supply  circuits    are  connected    through    transformers  A, 
J3,   C,  which  reduce  the  pressure  to  an  amount  which  is  suitable  for 
lamps  or  motors,  and  which  renders  the   conductors   safe  to  handle. 
The  transformer  B  has  its  secondary  connected  so  that  it  can  be  used 
on  a  three-wire  system.     Distributing  systems   for  polyphase  currents 
are  similar  in  essential  features  of  construction  to  the  single-phase  con- 
struction, except  that  three  or  four  wires  must  be  used  and  two  or  three 
transformers  must  be  erected  at  .each  point  of  transformation.1 

338.  High-pressure  Long-distance  Transmissions.  —  The  last  articles 
refer  especially  to  the  distribution  of  electrical  energy  over  limited  areas 
such  as  the  regions  of  electrical  'supply  in  individual  towns.     Within 
recent  years  it  has  been  found  possible  to  very  greatly  increase  the 
distance  of  transmission  by  increasing  the  pressure,  so  that  now  many 

1  This  is  described  more  fully  in  Article  246. 


ELECTRICAL  DISTRIBUTION  OF  POWER 


395 


plants  are  operating  which  transmit  power  over  distances  up  to  eighty  or 
more  miles  at  pressures  ranging  from  10,000  to  60,000  volts.  Recently, 
plants  have  been  planned  to  transmit  power  at  even  higher  pressures. 
There  is  every  reason  to  suppose  that  both  the  pressure  and  distance  will 
be  still  further  increased  during  the  next  decade. 

Much  trouble  has  been  experienced  in  properly  insulating  the  lines 
which  are  subjected  to  such  high  voltages.  If  the  insulators  become 
faulty,  leakage  of  the  current  is  apt  to  result,  which  chars  and  injures 
the  wooden  pins,  cross  arms,  and  poles,  as  well  as  causes  trouble  at  the 


ALTERNATOR 


THREE-WIRE  SECONDARY  DISTRIBUTION 


LLJ 

rl 


PRIMARY  DISTRIBUTING  CIRCUIT  OF  HIGH  PRESSURE 


LJ 


FlG.  297.  —  Diagrammatic  Illustration  of  Alternating  Current  Circuits  in  which  a  High 
Pressure  is  distributed  from  the  Generator  and  tianslormed  into  a  Safe  Pressure  at 
Centres  of  Use.  The  dots  represent  incandescent  lamps. 

generating  station,  due  to  short  circuits.  A  large  triple  petticoat  insu- 
lator of  porcelain  which  is  used  on  many  transmission  lines,  including 
the  one  which  reaches  from  Niagara  Falls  to  Buffalo,  is  illustrated  in 
Figure  298.  Insulators  of  this  character  are  sometimes  made  as  large  as 
ten  inches  across.  The  conductor  is  laid  in  the  groove  at  the  top  of 
the  insulator  and  tied  in  position  by  a  tie  wire  twisted  around  the  neck 
of  the  insulator. 

Another  difficulty  that  is  encountered  in  the  very  high-pressure  trans- 
mission is  the  prevention  of  loss  due  to  static  discharges  which  extend 


39^ 


ELECTRICITY  AND   MAGNETISM 


from  conductor  to  conductor  through  the  air.  The  loss  from  this  cause, 
which  may  be  considerable  unless  the  wires  are  several  feet  apart,  has 
not  as  yet  been  overcome.  Insulating  the  wires  with  a  continuous  cover- 
ing is  of  comparatively  little 
service,  and,  in  fact,  the  con- 
ductors of  such  lines  are  usu- 
ally bare. 

The  currents  for  long-dis- 
tance transmission  of  power 
by  electricity  are  usually  gen- 
erated at  ordinary  pressures, 
and  the  high  transmission 
pressures  are  obtained  by 
means  of  "  step-up  "  trans- 
formers at  the  power  house, 
which  increase  the  pressure 
of  the  generator  to  that  re- 
quired for  the  line.  This  is 
again  reduced  in  sub-sta- 
tions or  transformer  houses  at  points  where  the  energy  is  to  be  used. 
The  distribution  from  the  sub-station  to  the  users  is  similar  to  that 
described  in  the  last  article.  Three-phase  systems  are  ordinarily  used 
for  work  of  this  class,  though  the  three-phase  current  is  often  transformed 
into  two-phase  at  the  receiving  end  by  proper  transformer  combinations. 
With  increasing  knowledge  gained  by  experience  and  scientific  discov- 
ery it  will  become  possible  to  increase  the  pressure  to  higher  and  higher 
values,  and  thus  it  will  become  more  and  more  economical  to  transmit 
power  from  natural  water-power  sources,  coal  mines,  and  gas  countries 
by  electrical  means  to  very  widely  situated  centres.  This  development 
points  out  a  road  which  apparently  leads  to  wonderful  applications  of 
electrical  power. 

339.  Fire  Underwriters'  Rules.  —  The  importance  of  using  the  utmost 
care  in  laying  out  and  putting  in  place  the  electric  light  wires  which  go 
into  houses  is  reasonably  well  recognized.  It  now  comes  to  an  explana- 
tion of  the  more  important  rules  for  this  work  which  have  been  issued  by 
various  associations  of  fire  insurance  companies  or  underwriters.  These 


FlG.  298. —  Porcelain  Insulator  for  Use  with  High- 
pressure  Circuits  for  the  Electrical  Transmis- 
sion of  Power  over  Long  Distances. 


ELECTRIC   LIGHT  AND   POWER   WIRING  397 

associations  issue  rules  for  carrying  on  electrical  wiring,  and  in  the  large 
cities  supervise  or  inspect  the  work  in  order  that  danger  from  fire  may 
not  be  introduced  into  buildings  insured  by  them.  Many  of  the  chances 
for  accident  which  exist  in  electric  plants  are  caused  through  carelessness 
or  lack  of  knowledge  on  the  part  of  inexperienced  wiremen  who  may 
be  employed  on  account  of  the  false  economy  of  the  owner  of  the  plant. 
In  electrical  wcrk,  the  cheapest  is  by  no  means  always  the  best ;  but  it 
is  often  difficult  to  make  this  fact  apparent  to  the  owner  of  a  building 
who  must  pay  for  expensive  wiring,  and  a  carefully  enforced  set  of  rules 
for  the  wiring  is  the  best  safeguard  which  the  owners  of  buildings  and 
the  underwriters  have  against  dangers  caused  by  careless  workmen  and 
poor  workmanship. 

The  following  points  require  to  be  specially  looked  after  :  — 

1.  That  the  general  workmanship  is  good,  and  especially  that  joints 
in  conductors  (which  should  be  as  few  as  possible)  are  well  made  and 
well  insulated. 

2.  That  the  conductors  have  ample  cross  section. 

3.  That  the  insulating  material  on  the  conductors  is  of  the  very  best, 
and  that  the  insulation  resistance  of  the  completed  wiring  is  high. 

4.  That  absolutely  no  portion  of  the  electric  circuit  touches  any  part  of 
the  building,  but  is  separated  at  every  point  by  porcelain  cleats,  knobs, 
or  tubes,  or.  by  conduit  and  moulding. 

5.  That  the  insulation  resistance  of  the  wiring  is  tested  from  year  to 
year  to  ascertain  whether  or  not  it  is  deteriorating. 

6.  That  all  constant  pressure  circuits  are  properly  protected  by  safety 
fuses. 

By  insulation  resistance  is  meant  the  resistance  as  measured  from 
either  of  the  conductors  of  the  plant  to  the  ground,  or  from  one  con- 
ductor to  the  other.  Practical  methods  for  making  insulation  tests  will  be 
explained  in  the  next  chapter.  The  actual  resistance  of  the  insulation 
on  the  wiring  in  any  particular  building  always  depends  upon  the  length 
of  wire,  number  of  lamps,  and  character  of  the  fixtures  used  in  the  instal- 
lation. Thus,  for  instance,  if  wire  is  used  which  has  an  insulation  resist- 
ance of  1500  megohms  per  mile,  and  ten  miles  are  used,  the  total 
insulation  resistance  of  the  wire  cannot  be  expected  to  be  more  than 
150  megohms;  while  if  only  two  or  three  miles  of  wire  are  used,  the 


398  ELECTRICITY   AN7D    MAGNETISM 

total  insulation  resistance  may  be  expected  to  be  greater.  As  a  gen- 
eral rule,  leakage  at  joints,  lamp  sockets,  fuse  blocks,  and  fixtures  of  all 
kinds,  has  a  much  more  marked  effect  on  the  insulation  resistance  of 
new  wiring  than  does  the  leakage  through  the  covering  of  the  wire  itself, 
so  that  the  underwriters  require  these  points  to  be  specially  well  looked 
after.  It  is  usual  to  expect  a  much  higher  insulation  in  wiring  before 
the  sockets  and  fixtures  are  connected  up  than  afterward,  and  in  some 
places  the  insulation  resistance  which  is  required  in  any  plant  is  allowed 
to  depend  upon  the,  number  of  lamps  which  are  connected  to  the 
wires. 

Unless  the  best  of  materials  and  workmanship  are  used  for  the  wiring 
put  in  a  building,  the  insulation  resistance  will  begin  to  fall  within  a  few 
months,  even  though  it  was  very  high  when  the  wiring  was  first  put  in. 
This  fall  in  the  quality  of  the  insulation  is  due  to  several  causes,  chief 
among  which  are  poorly  insulated  joints  and  inferior  rubber  in  the  cover- 
ing of  the  wires.  Portions  of  wiring  which  have  been  in  service  from  a 
few  months  to  a  few  years  have  been  found  to  be  so  deteriorated  that  in 
certain  spots  the  rubber  coverings  on  the  wires  have  practically  all  rotted 
away.  It  is  sufficient  to  say  that  good  rubber-covered  wire  does  not  fail 
in  this  manner.  On  account  of  the  deterioration  of  poor  material,  an 
inspection  is  made  of  wiring  from  time  to  time  in  some  cities  ;  and  if  any 
short  branch  or  "  tap  "  falls  below  100,000  ohms  in  insulation,  measured 
between  the  wires  and  the  ground  or  between  the  wires  themselves,  it 
must  be  repaired. 

340.  Use  of  Safety  Fuses.  —  It  is  necessary  to  use  safety  fuses  on  all 
constant  pressure  circuits.  Safety  fuses  must  be  of  such  a  capacity  that 
they  will  blow,  or  melt,  just  above  the  rated  carrying  capacity  of  the 
smallest  wire  which  /hey  protect.  It  is  customary  to  place  fuses  at 
every  point  where  a  change  is  made  in  the  size  of  wire,  even  where  small 
fixtures  or  drop  cords  are  attached  to  the  tap  lines,  and  not  more  than  a 
dozen  sixteen-candle-power  incandescent  lamps  are  permitted  on  one 
"tap,"  except  under  special  conditions. 

Automatic  Circuit  Breakers  are  often  used  in  place  of  fuses  at  impor- 
tant points,  and  they  are  commonly  used  for  the  protection  of  the 
generators  against  excessive  loads  in  railway  power  stations  and  some 
electric  light  stations.  They  consist  of  switches  which  are  caused  to 


ELECTRIC   LIGHT  AND   POWER  WIRING 


399 


automatically  open  the  circuit  by  means  of  a  spring  and  trigger  actuated 
by  an  electromagnet  when  the  current  exceeds  a  proper  value. 

341.  Safe  Carrying  Capacity  of  Wires.  —  In  general,  the  size  of  a  fuse 
depends  upon  the  size  of  the  smallest  conductor  it  protects,  and  not 
upon  the  amount  of  current  to  be  used  in  the  circuit.  Below  is  a  table 
showing  the  safe  carrying  capacity  of  copper  conductors  of  different  sizes 
in  B.  &  S.  gauge,  as  given  in  the  generally  accepted  rules  issued  by  the 
National  Board  of  Fire  Underwriters  :  — 


SIZE  OF  WIRE 
B.  &  S.  Gauge 

CURRENTS  IN  AMPERES  WHICH  IT  is  SAFE  FOR  INTERIOR 

WIRES  TO  CARRY  CONTINUOUSLY 

Rubber  covered 
(Open  work  or  concealed) 

Weather-proof  insulation 
(Open  work) 

0000 

210 

3I2 

ooo 

I77 

262 

oo 

15° 

220 

0 

I27 

I85 

I 

I07 

I56 

2 

90 

131 

3 

76 

1  10 

4 

65 

92 

5 

54 

77 

6 

46 

65 

8 

33 

46 

10 

24 

32 

12 

17 

23 

14 

12 

16 

16 

6 

8 

18 

3 

5 

The  safe  carrying  capacities  of  insulated  aluminum  wires  are  84  per 
cent  of  the  capacities  for  copper  wires  which  are  given  in  this  table. 

By  "  open  work  "  in  this  table  is  meant  construction  which  admits  of 
all  parts  of  the  surface  of  the  insulating  covering  of  the  wire  being  sur- 
rounded by  free  air.  The  carrying  capacities  of  Nos.  16  and  18  wire  are 
given,  but  no  wire  smaller  than  No.  14  is  to  be  used,  except  for  the 
wiring  of  metal  fixtures  which  support  incandescent  lamps  under  ^ 
proper  conditions. 

UN  :Y   1 

it 


400  ELECTRICITY   AND    MAGNETISM 

342.  National  Code  of  Rules.  —  Until  within  the  last  few  years,  no 
uniformity  existed  in  the  rules  which  were  in  force  in  different  parts  of 
the  country,  but  the  associations  of  the  underwriters  located  in  various 
cities  or  districts  made  their  own  rules.  This  resulted  in  much  annoy- 
ance, and  did  not  tend  to  produce  the  best  workmanship ;  and  it  was 
found  to  be  of  advantage  to  formulate  a  satisfactory  set  of  rules  for  gen- 
eral adoption,  which  was  done  by  a  committee  of  the  underwriters  work- 
ing in  harmony  with  various  societies  of  electrical  engineers  and  electric 
light  experts. 

The  set  of  rules  thus  approved  is  called  the  National  Electrical  Code. 
Printed  copies  of  the  Code  can  be  obtained  from  the  local  inspectors,  or 
from  the  secretary  of  the  National  Board  of  Underwriters  in  Chicago. 

The  approved  rules  divide  electric  light  and  power  circuits  into  six 
classes  :  — 

1.  The  circuits  inside  of  central  stations  and  the  dynamo  rooms  of 
isolated  plants. 

2.  Constant  current  circuits,  usually  constructed  for  the  purpose  of 
operating  arc  lamps  or  other  devices  in  series. 

3.  High- pressure  circuits,  which  are  usually  alternating  current  lines. 

4.  Low-pressure  circuits,  which  include  all  low-pressure  inside  wiring 
and  some  outside  lines.      (Low-pressure  circuits  are  taken  to  include  all 
circuits,  except  grounded  electric  railway  circuits,  on  which  the  pressure 
does  not  exceed  550  volts.) 

5.  Grounded  electric  railway  circuits. 

6.  Extra  high  pressure  circuits.      (These  include  all  circuits  of  over 
3500  volts  pressure.) 

For  each  of  these  classes  of  circuits,  special  rules  are  directed  toward 
the  perfect  safety  of  the  systems  which  especially  emphasize  the  very 
essential  points  referred  to  above.  Every  rule  has  a  good  reason  for  its 
existence,  and  experience  has  shown  its  propriety.  Circuits  of  the  fifth 
and  sixth  classes  are  refused  admission  to  any  buildings  for  the  purpose 
of  furnishing  electric  light  and  power  except  in  the  power  houses,  sub- 
stations, and  car  barns  of  the  electric  companies. 

The  only  classes  of  wiring  with  which  the  underwriters'  rules  do  not 
deal  directly  are  connected  with  telephone,  district  messenger  call,  burg- 
lar alarm,  electric  bell,  and  similar  systems  which  are  operated  by  elec- 


ELECTRIC   LIGHT  AND    POWER   WIRING  40 1 

trie  batteries.  Even  in  regard  to  these  wires  the  rules  enjoin  proper 
precautions  to  prevent  electric  light  and  power  wires  from  becoming 
crossed  with  the  poorly  insulated  battery  circuit  wires. 

343.  Wiring  Buildings.  —  The  importance  of  the  parts  of  electric 
lighting  circuits  which  are  inside  of  buildings  cannot  be  overestimated. 
A  central  station  may  be  built  upon  the  best  plan  to  supply  current 
through  a  perfect  distributing  system,  but  a  safe  and  satisfactory  light 
will  not  be  given  if  the  Inside  Wiring  is  poorly  planned  and  badly  put  in 
place.     Fires  which  occur  on  account  of  the  electric  light  wires  in  houses 
are  always  caused  either  by  the  use  of  poor  material,  careless  planning, 
or  bad  workmanship  when  the  inside  wiring  was  put  in,  and  if  the  iviring 
is  done  properly,  it  is  almost  impossible  for  fires  to  be  caused  by  an  electric 
lighting  or  power  system.     On  the  other  hand,  poorly  constructed  wiring 
should  not  be  permitted  anywhere. 

On  account  of  the  danger  which  may  be  caused  by  unscrupulous  or 
untrustworthy  vviremen,  it  is  usual  in  large  cities  to  have  official  inspect- 
ors who  examine  and  test  all  electric  light  work  placed  within  buildings. 
It  is  the  duty  of  these  inspectors  to  see  that  the  work  is  safely  and  prop- 
erly done  in  accordance  with  rules  fixed  by  the  city  authorities  and 
approved  by  the  fire  underwriters.  Even  with  such  inspection  the  work 
is  not  always  done  in  the  best  manner ;  yet  comparatively  few  important 
fires  have  been  caused  by  electric  wires,  and  those  have  usually  been  the 
result  of  dense  ignorance  or  worse,  —  carelessness.  A  great  majority  of 
the  accidents  laid  to  the  door  of  electricity  are  due  to  some  other  cause. 

For  ordinary  wiring  inside  of  a  building  only  the  very  best  rubber-cov- 
ered wire  should  be  used.  A  great  many  factories  produce  rubber-cov- 
ered wire  for  use  in  inside  wiring,  and  much  of  it  is  very  poor,  so  that 
considerable  care  is  necessary  in  selecting  material. 

344.  Cleat  and  Moulding  Work.  —  The  wires  may  be  run  in  buildings 
according  to  three  entirely  different  methods.     In  the  first  place,  the 
wires"  may  be  run  upon  the  surfaces  of  ceilings  or  walls  in  plain  sight, 
where  they  are  held  in  place  by  means  of  cleats  made  of  porcelain,  or, 
if  more  convenient,  porcelain  knobs  may  be  used.     This  is  a  common 
arrangement  of  wiring  in  small  stores  and  other  buildings  where  the  posi- 
tion of  the  wires  in  plain  sight  is  not  objectionable.     As  the  wires  are  all 
visible  and  therefore  can  be  easily  inspected  at  any  time,  Open  Work 

2D 


402 


ELECTRICITY   AND    MAGNETISM 


or  Cleat  Work,  as  this  arrangement  is  called,  is  a  safe  and  satisfactory 
arrangement  of  wiring,  provided  the  wires  and  appliances  are  all  out  of 
reach  so  that  they  cannot  be  tampered  with. 

There  are  many  places  where  the  appearance 
of  open  work  is  objectionable,  but  where  the 
wires  may  be  placed  in  wooden  casings  or 
Mouldings  which  are  fastened  to  ceilings  or 
walls  in  plain  sight.  This  is  an  exceedingly 
safe  and  satisfactory  arrangement,  since  the 
wires  are  well  protected  from  mechanical  in- 
jury or  from  being  tampered  with,  and  yet  the 
condition  of  the  wiring  may  be  easily  seen  at 
any  time  by  a  simple  inspection.  Common 
forms  of  mouldings  are  shown  in  Figure  299. 
These  may  be  made  of  any  desired  wood, 
though  hard  pine  is  most  commonly  used. 

345.  Concealed  Work.  —  In  the  third  method 
of  running  wires  they  are  placed  entirely  out 
of  sight,  or  Concealed.  This  may  be  done  in 
various  ways.  The  oldest  and  at  the  same 
time  the  least  safe  and  satisfactory  way  was  to 
fasten  the  wires  to  the  ceilings  and  walls  of  the 
building  before  the  plastering  was  put  on.  The 
wires  were  then  entirely  covered  by  or  em- 
bedded in  the  plaster,  so  that  it  was  impossible 
to  examine  or  repair  them  without  injury  to  the 
walls ;  and,  indeed,  the  positions  of  the  wires 
in  the  walls  were  often  forgotten  in  a  few 
months  after  the  building  was  finished,  so  that 

repairs  were  doubly  difficult  to  make.  This  arrangement  of  the  wires 
was  made  more  unsafe  because  the  plaster  upon  the  walls  often  spoiled 
the  insulating  qualities  of  the  rubber  coverings,  and  the  wires  became 
"grounded"  as  a  consequence.  The  arrangement  is  no  longer  per- 
mitted by  the  underwriters. 

In  buildings  with  wooden  floors  and  partitions  it  is  permissible  to  fasten 
the  wires  to  the  floor  joists  or  partition  studding  by  means  of  porcelain 


FlG.  299.  —  Samples  of 
Wood  Mouldings  for 
Electric  Light  Wires. 
These  samples  are 
shown  nailed  to  a 
board  for  exhibition. 


ELECTRIC   LIGHT  AND    POWER   WIRING  403 

cleats  or  knobs  in  such  a  way  that  the  wires  do  not  touch  anything  except 
their  insulating  supports,  while  porcelain  or  other  tubes  surround  the  wires 
wherever  they  pass  through  walls  or  joists.  When  this  is  properly  done, 
ihe  wires  are  not  likely  to  be  injured  by  plaster  or  dampness,  but  the 
disadvantage  that  they  cannot  be  examined  is  still  present.  They  are 
also  liable  to  injury  by  plumbers,  carpenters,  or  other  workmen  who  are 
engaged  in  making  repairs  or  alterations  to  the  building. 

It  is  much  better  to  arrange  a  hidden  conduit  to  contain  the  concealed 
wires.  This  may  be  placed  behind  decorations  or  other  objects  on  the 
walls  or  may  be  laid  neatly  under  the  floors  or  the  plaster.  Special 
tubes  are  made  to  be  used  as  "  conduits  "  for  inside  wiring.  These  are 
called  "  interior  conduit,"  "  loricated  pipe,"  and  other  trade  names,  and 
they  are  used  to  a  large  extent.  They  are  essentially  nothing  more  than 
strong,  smooth,  water-tight  iron  tubes,  composed  of  iron  pipes  with  var- 
nished or  enamelled  interiors.  Interior  tubing  made  of  insulating  mate- 
rial was  originally  intended  to  take  the  place  of  the  rubber  insulation  on 
the  wires  so  that  they  could  be  used  with  a  cheap  cotton  covering,  but 
it  was  found  to  be  necessary  to  use  the  best  rubber  insulation  on  wires 
in  the  tubes  in  order  that  the  wiring  might  give  satisfaction,  and  the  insu- 
lating tubes  are  not  often  used  now.  The  advantages  of  tubes  lie  in  the 
fact  that  the  wires  are  protected  from  mechanical  injury  and  from  con- 
tact with  plaster,  moisture,  and  other  deleterious  agencies.  These  tubes 
should  be  so  constructed  that  the  wires  may  be  readily  pulled  into  or 
out  of  them  at  any  time,  so  that  alterations  or  repairs  may  be  made 
whenever  required. 

346.  Distributing  Systems  for  Wiring  Buildings. — The  plan  of  the 
wiring  in  a  building  depends  a  great  deal  upon  the  size  and  construction 
of  the  building,  but  in  its  details  it  should  always  fulfil,  not  only  in  the 
letter  but  in  the  spirit,  the  requirements  of  the  underwriters  which  are 
laid  down  in  the  special  printed  rules  called  the  National  Electrical  Code. 
In  small  buildings  supplied  with  current  from  a  central  station  the  sim- 
plest plan  for  concealed  wiring  is  what  may  be  called  the  "  central  cabi- 
net plan."  Heavy  service  wires  are  led  from  the  street  mains  of  the 
electric  light  company  through  a  fuse  block  or  cut-out  (Fig.  300)  to  a 
convenient  central  point  in  the  building.  At  this  point  the  main  wires 
terminate  in  a  cabinet  which  contains  a  number  of  fuse  blocks  from  each 


404 


ELECTRICITY   AND   MAGNETISM 


of  which  a  circuit  of  smaller  wires  runs  out  to  supply  a  limited  number 
of  lamps,  which  is  usually  between  5  and  15.     Figure  301  shows  such  a 

plan  of  wiring  so  plainly 
that  an  extended  de- 
scription is  not  neces- 
sary. In  the  figure,  S, 
S,  S,  are  switches  for 
turning  the  lights  on 
and  off,  and  C  is  a  fuse 
block  used  to  protect 
a  small  branch  circuit 

FIG.  300. -Main  Fuse  Block.  which    for    convenience 

is  connected  to  one  of 

the  Taps  instead  of  being  run  back  to  the  distributing  centre.     By  this 
arrangement  of  the  distribution  any  serious  trouble  which  occurs  on  one 


FIG.  301. —  Diagrammatic  Scheme  of  Wiring  for  a  Residence. 

branch  or  tap  causes  the  fuses  at  the  distributing  centre  which  belong  to 
the  branch  to  melt.      This  disconnects  the  defective  branch  from  the 


ELECTRIC   LIGHT  AND    POWER   WIRING 


405 


service  wires  without  interfering  with  the  other  branches.  The  location 
of  all  the  fuse  blocks  at  a  central  point  makes  it  convenient  to  replace 
fuses,  and  the  fuse  blocks  can  be  so  protected  that  a  fire  cannot  pos- 
sibly be  caused  by  the  arc  which  sometimes  occurs  when  a  fuse  melts 
or  Blows. 

Another  plan  for  wiring  a  building  is  shown  in  Figure  302.  In  this 
figure  a  heavy  trunk  circuit  runs  from  the  main  cut-out  in  the  cellar  to 
the  top  of  the  house,  and  the  lamp  taps  branch  off  from  the  trunk  at 


FlG.  302.  —  Diagrammatic  Scheme  of  Wiring  for  a  Residence. 

each  floor.  S,  S,  S are  switches  for  controlling  the  lights,  and  C,  C,  Care 
fuse  blocks.  This  plan  makes  it  necessary  to  scatter  the  fuse  blocks 
through  different  parts  of  the  house,  which  is  a  disadvantage. 

In  large  buildings  a  combination  of  the  two  plans  just  explained  is 
used,  and  feeding  trunks,  or  feeders,  are  run  from  the  main  fuse  block 
to  several  distributing  centres  at  convenient  points  in  the  building.  One 
feeder  with  its  mains  is  illustrated  in  Figure  303,  where  AB  is  the  feeder 
running  from  the  main  cut-out,  or  from  the  dynamo  room  if  a  special 
lighting  plant  is  located  in  the  building,  and  C,  C,  C,  C  is  a  main  which 


406 


ELECTRICITY   AND   MAGNETISM 


runs  down  and  up  so  as  to  supply  current  to  the  different  floors  of  the 
building.  Fuse  blocks  are  placed  at  each  rectangle  to  protect  the  parts 

of  the  circuit  beyond  it.  The  horizontal 
lines  are  mains  which  run  along  each 
floor  to  carry  current  to  the  distributing 
centres,  which  are  shown  by  the  rectangles 
at  X,  X,  X.  The  lamp  taps,  which  are 
run  from  the  centres  to  the  lamps,  are 
represented  by  the  short  lines  which  run 
out  from  the  rectangles.  A  dozen  or 

nil  I   H  more  taps  often   radiate    from   a   single 

XLJ n — v  r-ax  nx  nx  v        .  ,    .       .  . 

^ — =r       centre.     Only  one  line  is  used    in  this 

figure  to  represent  the  circuit,  which  may 
be  either  two-wire  or  three-wire. 

For  very  large  buildings  the  plan  shown 
in  Figure  303  may  be  extended  by  run- 
ning feeders  to  various  points  in  the 
building,  from  which  points  mains  run 
to  the  distributing  centres.  The  feed- 
ing points  are  then  usually  joined  to- 
gether by  a  heavy  connecting  circuit, 

often  called  a  Crib.  In  Figure  304,  At  A,  A,  A  are  feeders  running  to 
four  feeding  points  in  a  building,  which  are  marked  B,  B,  B,  B.  These 
points  are  joined  together  by  the  crib  from  which  the  mains  C,  C,  C,  C 
run  off  to  the  various  centres  of  distribution. 

347.  Sizes  of  Wire  for  Inside  Wiring.— The  wiring  plan  in  a  large 
building  is  seen  to  be  quite  similar  to  the  plan  of  the  feeders  and  mains 
used  in  distributing  electric  current  from  a  central  station.1  The  object 
to  be  aimed  at  in  arranging  the  wires  in  either  case  is  to  keep  all  of  the 
lamps  which  are  burning  at  one  time  at  as  nearly  as  possible  the  same 
pressure,  and  also  to  make  it  possible  to  keep  the  pressure  constant, 
regardless  of  the  number  of  lamps  burning.  The  size  of  wires  used  at  any 
place  must  be  calculated  from  the  amount  of  current  which  the  wires 
carry,  and  the  volts  drop  (or  loss  in  pressure)  which  is  allowed.  The  cal- 
culation sometimes  indicates  a  wire  which  is  too  small  for  safety,  as  a 

1  Article  328. 


303.  —  A  Feeder  and  Main 
System  for  Large  Buildings.  A 
single  line  is  used  to  represent 
both  conductors  of  the  circuit. 


ELECTRIC   LIGHT  AND   POWER   WIRING 


407 


wire  smaller  than  No.  14  B.  &  S.  gauge  should  never  be  used  for  inside 
electric  light  wiring ;  neither  should  the  current  passed  through  a  wire 
exceed  the  "  safe  carrying  capacity  "  given  in  the  table  which  is  printed 
in  Article  341.  "Wiring  tables,"  which  give  the  sizes  of  inside  wires 
which  are  required  to  supply 
current  to  lamps  at  various 
distances  from  the  main  cut- 
outs, are  to  be  found  in  many 
trade  catalogues  and  various 
small  special  books. 

348.  Fuses.  — A  great  many 
details  relating  to  inside  wir- 
ing can  only  be  learned  by  ob- 
serving wiring  which  has  been 
completed  in  a  proper  man- 
ner, but  a  great  deal  of  useful 
information  relating  to  the  in- 
cidental material  can  be  ob- 
tained from  the  catalogues  of 
the  companies  who  supply  elec- 


A     A 


FIG.  304. — Arrangement  of  Feeders  and  Mains 

for  Large  Buildings, 
trical  material.    The  most  im- 
portant incidental  material  consists  of  fuse  blocks,  fuses,  switches,  and 
sockets. 

Fuse  blocks  now  invariably  consist  of  porcelain  bases  of  various  forms 
upon  which  are  carried  terminal  screws  for  the  connection  of  the  fuses 
and  the  circuit  wires.  Electric  light  fuses  are  strips  or  wires  of  a  metal 


CjOPPER 


alloy  which  melts  at  a  temperature  that 
is  so  low  that  the  melted  metal  cannot 
possibly  cause  harm.  The  alloy  is  usu- 
ally made  largely  of  lead  and  tin,  but 
varies  a  great  deal. 

The  object  of  the  fuse  is  to  protect 
the  wires  beyond  it  from  becoming  over- 
heated through  some  accident.  The  size  of  the  fuse  at  any  point  is 
such  that  if  anything  occurs  to  cause  an  unsafe  current  to  flow  through 
the  wires  protected  by  it,  the  fuse  will  melt  and  cut  the  wires  out  of  the 


TERMINAL  TERMINAL 

FIG.  305.  —  Fuse  of  "Link"  Pattern, 
with  Copper  Terminals. 


408 


ELECTRICITY  AND    MAGNETISM 


circuit.  Fuses  of  large  carrying  capacity  are  composed  of  strips 
or  "links"  of  fuse  metal  which  are  tipped  at  each  end  by  a  ter- 
minal of  copper  (Fig.  305),  so  that  a  more  substantial  contact 


PORCELAIN  BODY: 


BRASS  BOTTOM  CONTACT' 


SCREW  SHAPED 
BRASS  SHELL 


PORCELAIN  BODY 


PORCELAIN  BODY 


CREW  SHAPED 
BRASS  SHELL 


BRASS  CAP  TO  SCREW 
OVER  PORCELAIN  BODY 


FIG.  306.  —  Edison  Plug  Fuse. 


may  be  made  with  the  fuse  block  terminals, 
are   also  made  of  the  "link"   pattern,    but 


Fuses  of  small  capacity 
plug   fuses,  such  as  are 
illustrated  in  Fig.  306,  are  very  commonly 
used  where  currents  do  not  exceed  thirty 
amperes. 

348  a.  Switches.  —  Each  of  the  house 
switches,  or  "  snap  "  switches,  that  are  now 
generally  used  to  control  electric  light  "  tap  " 
circuits,  consists  of  a  small  electric  circuit 
closer  arranged  with  a  spring  so  that  it 
opens  or  closes  the  circuit  with  a  snap  when 
the  handle  is  moved.  The  quick  action  pre- 
vents the  switch  points  from  becoming 
burned  by  the  spark  upon  opening  the  cir- 
cuit. A  switch  is  called  Double  Pole  when 
it  carries  separate  blades  which  simultane- 
ously control  the  two  wires  of  a  circuit.  A 
Single  Pole  switch  controls  only  one  wire  of 
a  circuit. 

The   blades   of  central    station   switches 

usually  make  a  rubbing  contact  between  substantial  leaves  of  copper 

when  the  circuit  is  closed  through  them. 


FIG.  306  a.  —  Edison  Plug  Fuse 
Block. 


ELECTRICAL  DISTRIBUTION   OF   POWER  409 

I 

QUESTIONS 

32.  What  is  a  series  system?     A  parallel  system? 

33.  How  should  the  drop  in  pressure  be  divided  between  feeders  and  mains  in 
a  parallel  system? 

34.  What  is  a  multiple  series  system? 

35.  What  are  the  advantages  and  disadvantages  of  the  multiple  series  system 
for  incandescent  lighting? 

36.  What  is  the  three-wire  system? 

37.  What  is  the  function  of  the  neutral  wire  in  a  three-wire  system? 

38.  Why  does  no  current  flow  to  or  from  the  dynamos  over  the  neutral  wire  of  a 
balanced  three-wire  system? 

39.  Give  a  hydraulic  analogy  to  the  three-wire  system. 

40.  How  much  copper  is  saved  by  the  three-wire  system? 

41.  What  is  a  five-wire  system?     How  much  copper  is  saved  by  it? 

42.  Describe  an  alternating  current  system. 

43.  What  are  the  advantages  of  alternating  currents  for  electric  transmission  of 
power? 

44.  Over  what  extremes  of  distance  has  electric  power  been  transmitted  by  means 
of  alternating  currents?     At  what  pressures? 

45.  What  kind  of  insulators  are  used  for  very  high  pressure  lines? 

46.  What  is  a  "step-up"  transformer? 

47.  Describe  a  high-pressure  transmission  system. 

48.  Why  are  underwriters'  rules  necessary? 

49.  What  points  must  be  especially  looked  after  in  the  electric  wiring  of  houses? 

50.  Why  are  poorly  constructed  sockets,  fuse  blocks,  and  fixtures  particularly 
likely  to  be  the  seats  of  bad  insulation? 

51.  What  rules  are  now  generally  used  to  govern  wiring? 

52.  Why  must  safety  fuses  be  made  to  protect  the  smallest  wire  in  a  circuit? 
Will  larger  wire  in  the  same  circuit  then  be  protected? 

53.  Is  it  permissible  for  any  part  of  the  wiring  of  a  building  to  rest  against  the 
building  materials? 

54.  What  should  be  the  minimum  insulation  resistance  of  individual  taps? 

55.  How  much  current  will  a  No.  oooo  wire  safely  carry?     How  much  a  No.  12? 

56.  What  kind  of  wire  should  be  used  for  inside  work? 

57.  How  is  cleat  and  moulding  work  executed? 

58.  How  is  concealed  work  executed? 

59.  What  is  interior  conduit  ?     How  is  it  used? 

60.  How  is  the  feeder  and  main  arrangement  used  for  interior  wiring? 

61.  What  is  the  crib  system  of  interior  wiring? 

62.  Where  and  how  is  it  desirable  to  arrange  the  fuses  in  a  building? 

63.  Describe  a  fuse  and  a  fuse  block. 


CHAPTER  XXI 

APPLICATIONS  OF  ELECTRICAL  INSTRUMENTS  TO  THE  TESTING 
OF  LINES  AND  CIRCUITS 

MEASUREMENTS  OF  ILLUMINATION 

349.  Troubles  in  Telegraph  and  Telephone  Lines.  —  On  account  of 
their  exposed  positions,  overhead  telegraph  and  telephone  wires  are  par- 
ticularly liable  to  injury.  It  is  therefore  necessary  to  make  careful,  sys- 
tematic, and  continued  tests  of  important  lines  in  order  that  they  may 
be  kept  in  satisfactory  condition.  The  troubles  to  which  lines  are  heir 
may  be  divided  into  four  classes  :  — 

1.  Grounds. 

2.  Crosses. 

3.  Poor  connections. 

4.  Breaks. 

A  line  is  said  to  be  Grounded  when  so  much  current  leaks  from  it 
to  the  ground  as  to  interfere  with  its  proper  use.  Grounding  may  be 
caused  by  a  general  leakage  all  along  the  line,  or  a  large  leak  may  exist 
at  one  point,  where  the  line  comes  in  contact  with  trees,  etc.  When  the 
insulation  of  the  line  becomes  so  -low  that  practically  all  the  current 
leaks  off,  it  is  said  to  be  Dead  Grounded. 

Lines  are  said  to  be  Crossed  when  they  make  contact  with  each  other 
so  that  current  sent  over  one  line  may  stray  on  to  the  other.  When  tele- 
graph or  telephone  lines  are  crossed,  only  one  of  them  can  be  used  to 
send  independent  messages,  since  the  messages  sent  over  one  line  may 
be  received  on  the  others,  and  if  it  is  attempted  to  use  the  several  lines 
at  the  same  time,  the  various  messages  become  badly  mixed  up. 

The  most  fruitful  cause  of  crosses  is  the  swinging  of  loose  wires  in  the 
wind,  by  which  means  they  become  tangled  up.  Sometimes  crosses  or 
grounds  will  appear  and  disappear  at  intervals,  when  they  are  often 
called  Swinging  Crosses  or  Grounds.  These  may  be  caused  by  a  swing- 

410 


LINE  TESTING  -  ,         411 

ing  wire  which  touches  another  wire  or  a  ground  contact  at  intervals,  but 
does  not  remain  continuously  in  contact. 

Poor  Connections  result  from  various  causes,  such  as  corroded  joints  in 
the  wire,  a  corroded  connection  to  a  ground  plate  or  water-pipe,  a  poor 
contact  between  the  ground  plate  and  the  earth,  loose  connections  at 
binding  posts  of  instruments,  at  switchboards,  or  at  batteries.  Poor 
connections  may  very  seriously  reduce  the  conductivity  of  the  line,  and 
thus  reduce  the  distinctness  of  messages  sent  over  it  unless  extra  battery 
power  is  used. 

A  Break  may  be  caused  by  a  binding  post  connection  working  entirely 
loose,  by  a  wire  breaking  at  an  instrument,  or  by  the  line  wire  breaking. 
It  may  also  be  caused  by  defective  contacts  in  the  working  parts  of  an 
instrument,  or,  in  the  case  of  a  telegraph  line,  by  a  careless  operator 
leaving  his  key  open.  When  a  line  wire  breaks,  the  circuit  may  be 
entirely  opened,  or  if  one  or  both  of  the  ends  get  on  the  ground,  it  may 
be  possible  to  get  current  through  one  or  both  portions. 

350.  Simple  Tests  of  Telegraph  Lines. — The  simplest  way  of  deter- 
mining the  condition  of  a  line  is  by  comparing  the  distinctness  of  the 
signals  which  are  transmitted  over  it  from  day  to  day.  Thus,  in  the  case 
of  a  telegraph  line,  if  signals  going  out  from  a  terminal  station  where  half 
the  battery  is  located  are  found  to  be  strong  and  good  on  a  certain  day, 
while  signals  coming  into  the  same  station  over  the  same  wire  are  weak 
and  indistinct,  it  is  evident  that  the  insulation  of  the  line  is  poor.1  If 
the  signals  which  are  sent  and  received  are  equally  indistinct,  while  the 
battery  is  in  good  condition,  the  conductivity  of  the  line  is  probably  less 
than  usual.  If  signals  sent  over  one  wire  can  be  received  on  another, 
the  lines  are  either  crossed  or  sufficient  current  leaks  from  one  wire  to 
the  other  to  give  the  effect  of  a  cross.  In  the  case  of  a  break  which 
opens  the  circuit,  the  armatures  of  the  relays  in  the  line  fall  back  from 
their  magnets  ;  but  if  the  ends  of  the  line  at  the  break  become  grounded, 
it  may  be  possible  to  send  signals  between  stations  upon  the  same  side 
of  the  break. 

The  section  between  two  stations  upon  which  Trouble  exists,  may  be 
readily  located  in  the  case  of  a  local  telegraph  line  passing  through  sta- 
tions which  are  close  together.  To  do  this,  the  station  nearest  one  end 

1  Compare  Article  325. 


412  ELECTRICITY   AND   MAGNETISM    • 

is  called  up  from  the  end  station,  either  by  means  of  the  faulty  wire  or 
by  means  of  another  wire,  and  is  told  to  ground  the  faulty  wire.  This 
being  done,  signals  are  transmitted  between  the  two  stations  over  the 
faulty  wire.  If  the  wire  works  all  right,  the  next  station  is  called  up  and 
the  test  of  the  working  condition  of  the  wire  is  again  made.  This  is 
continued  from  station  to  station  until  the  signals  fail  in  transmission. 
The  trouble  is  then  on  the  last  section  tested,  and  a  lineman  may  be 
sent  out  to  locate  it  exactly  and  correct  it. 

351.  Tests  of  Telephone  Lines. — Trouble  on  telephone  lines  is  usu- 
ally shown  to  the  exchange  operator  through  difficulty  or  impossibility  in 
communicating  with  a  subscriber.    Crosses  may  make  themselves  evident 
through  the  fact  that  when  a  subscriber  on  one  of  the  crossed  wires  calls 
the  exchange  by  working  his  magneto,  not  only  does  the  drop  fall  which 
is  attached  to  his  wire,  but  the  drops  which  are  attached  to  the  wires 
which  are  crossed  with  his  also  fall.     What  is  known  as  Cross  Talk 
between  telephone  wires  is  not  a  certain  sign  of  a  cross,  as  it  may  be 
caused  by  either  electromagnetic  or  electrostatic  induction.1     The  latter 
is  a  very  common  cause  of  cross  talk.     To  avoid  cross  talk  in  telephone 
cables,  the  two  wires  of  each  metallic  circuit  are  twisted  together,  and 
such  a  cable  is  therefore  said  to  be  made  up  of  Twisted  Pairs. 

352.  Tests  with  Instruments.  —  In  the  case  of  long  Trunk  telegraph 
or  telephone  lines  connecting  cities  at  a  considerable  distance  apart,  meth- 
ods are  required  for  the  location  of  faults  by  direct  electrical  measure- 
ments.    It  is  usual  to  make  careful  daily-  or  weekly  measurements  of  the 
insulation  and  conductivity  and  sometimes  of  the  capacity  of  such  lines. 
The  results  of  these  measurements  are  carefully  recorded  in  a  book, 
and  the  records  are  a  material  aid  in  the  location  of  faults  by  electrical 
measurement.     The  usual  instruments  to  be  used  in  testing  lines  are  a 
sensitive  galvanometer2  and  a  Wheatstone  bridge.3 

353.  Line  Conductivity.  —  To  measure  the  conductivity  of  a  metallic 
circuit  is  very  simple.     At  one  end  of  the  line  the  two  wires  are  con- 
nected together,  and  at  the  other  end  of  the  line  the  two  wires  are  con- 
nected to  the  bridge  (Fig.  307).     Half  the  resistance  measured  by  the 
bridge  is  the  resistance  of  one  of  the  wires  composing  the  circuit,  if  the 
wires  are  of  equal  length  and  size. 

1  Articles  137  and  190.  "  Article  146.  3  Article  162. 


LINE  TESTING 


413 


FlG.  307.  —  Diagram  showing  the  Connection 
of  Two  Telegraph  Lines,  or  a  Metallic 
Circuit  Telephone  Line,  with  Wheatstone 
Bridge  for  the  Purpose  of  measuring  their 
Joint  Resistance. 


When  only  one  wire  is  available,  as  may  be  the  case  with  telegraph 
circuits,  its  far  end  is  connected  to  ground,  the  near  end  is  connected 
to  one  of  the  binding  posts  of 
the  bridge,  and  the  other  bind- 
ing post,  to  which  the  unknown 
arm  should  be  connected,  is  con- 
nected to  ground,  as  shown  at 
E  in  Figure  308,  which  shows 
the  arrangements  of  the  connec- 
tions for  a  post-office  pattern 
bridge.1  With  this  arrangement, 
the  arm  of  the  bridge  marked 
A  in  the  diagrams,  such  as  Figures  307  and  309^  is  between  A  and  B. 
The  arm  B  is  between  B  and  C ;  the  arm  R  is  between  A  and  D  in 
the  zigzag  part  of  the  bridge  rheostat ;  and  the  unknown  resistance  is 

connected  to  the  bridge  at  the  points 
C  and  D.  The  resistance  measured 
by  the  bridge  may  be  taken  as  equal 
to  the  resistance  of  the  line,  provided 
the  ground  connections  are  good. 

354.  Line  Conductivity  --  Three 
Wires  Available.  —  When  the  indi- 
vidual conductivity  of  three  wires  run- 
ning between  the  same  points  is  de- 
sired, the  measurement  is  very  simple. 
The  resistances  of  the  wires  taken  in 
pairs  (Fig.  309)  is  measured  exactly 
as  in  the  case  of  a  metallic  circuit. 
From  these  measurements  the  resist- 
ance of  each  wire  may  be  calculated.  For  instance,  if  wires  i  and  2 
taken  together  measure  4500  ohms,  i  and  3  taken  together  measure 
3750  ohms,  and  2  and  3  taken  together  measure  4700  ohms,  then  the 
resistance  of  all  three  wires  in  series  would  be 

45QQ  +  375Q  +  47QQ  =  64?5 

2 
1  Article  164.  2  Also  see  Article  163. 


*"t" I 

r — r — i •! 


n 

GROUND  GROUND 

FlG.  308.  —  Diagram  showing  the  Con- 
nection of  One  Grounded  Telegraph 
or  Telephone  Line  with  Bridge  for 
the  Purpose  of  measuring  its  Re- 
sistance. 


414 


ELECTRICITY   AND   MAGNETISM 


Wire  No.  i  then  measures  the  difference  between  the  resistance  of  all 
in  series,  and  that  of  Nos.  2  and  3  together,  or  the  difference  between 

6475  and  4700-    No.  i,  there- 
fore, measures  1775  ohms.    In 
t  the  same  way  wires    Nos.   2 
N0. 2                   4)          and  3  are  found   to  respec- 
N0-  3 .  j          tively  measure  2725  and  1975 

FIG.  309.  —  Diagram  indicating  Connections  with     ohms. 
Bridge  where  Three  Wires  are  Available  and  it 
is  desired  to  obtain  the  Conductivity  of  Each. 


Example  A.     If  it  is  found  by 
bridge    measurement   that   the   re- 
sistance of  three  telegraph  wires  taken  together  in  pairs  are  3000,  4000,  and  5000 
ohms,  what  is  the  resistance  of  each  wire?     Ans.  3000,  2000,  and  1000  ohms. 

355.  Earth  Currents.  —  When  resistance  measurements  are  made  with 
the  earth  as  part  of  the  circuit,  currents  flowing  in  the  earth  may  inter- 
fere with  the  results  by  entering  the  wire  and  flowing  along  it.     Such 
currents   are   called   Earth   Currents.      At   exceptional   times,   as,    for 
instance,  during  the  continuance  of  the  so-called  Magnetic  Storms,  earth 
currents  flowing  on  the  wires  may  be  so  strong  that  telegraphing  may  be 
carried  on  without  any  battery  attached  to  the  wires. 

When  earth  currents  interfere  with  the  measurements  made  on  a 
grounded  circuit,  the  tests  must  be  postponed  until  a  more  favorable 
opportunity,  if  additional  wires  cannot  be  used  in  making  the  measure- 
ments by  the  last  method  given  above. 

356.  Line   Insulation.  —  Exact   insulation   measurements   are   made 
with  the  line  disconnected  from  its  ground  plates  (the  line  Open,  Figure 
310).    As  a  general  rule,  the  insulation  resistance  is  higher  than  an  ordi- 
nary Wheatstone  bridge  will 

measure,  and  the  method  ex- 
plained in  Article  169  is  used. 
The  condition  of  the  insu- 
lation of  a  line  may  also  be 
roughly  determined  from  day 
to  day  with  the  circuit  closed. 

A  milliamperemeter  is  placed  in  the  circuit  at  one  end  of  the  line  in 
series  with  a  battery  of  a  fixed  number  of  cells.  If  the  resistance  of 
the  circuit  and  the  pressure  of  the  battery  are  known,  a  certain  standard 


TO  MEASURING 
INSTRUMENTS 


J 


THIS  END  OPEN 


FIG.  310.  —  Telegraph  Line  with  Ends  "  Open"  for 
the  Purpose  of  making  Insulation  Measure- 
ments. 


LINE  TESTING  415 

current,  which  may  be  calculated  according  to  Ohm's  Law,1  should  flow 
through  the  line  when  the  insulation  is  perfect.  The  difference  between 
the  current  indicated  by  the  amperemeter  and  the  standard  current  is 
a  measure  of  the  leakage  from  the  line. 

A  comparison  between  the  recorded  periodical  measurements  of  con- 
ductivity and  insulation  shows  whether  or  not  the  line  is  in  good  order, 
or  whether  or  not  any  poor  connections  are  developing  or  its  insulation 
is  deteriorating. 

357.   Location  of  a  Ground.  —  The  location  of  the  position  of  a  ground 
or  a  cross  on  a  line  may  be  determined  in  various  ways.     If  the  fault 
is  a  "  dead  ground,"  a  measure- 
ment of  the  resistance  of  the  line  E 
and  ground   return  is  made  by 


"bridge"  from  one  end  of  the          .  —  ^(t)\  IEAKTO    *  <*•* 

line,  the  other  end  of  the  line 


being  open  (Fig.  311)  ;  and  the 
distance  to  the  "  ground  -  is  cal- 

culated  at  once  from  the  resist-  locating  the  Position  of  the  "  Fault  "  at  F. 

ance  of  the  line  per  mile.    Thus, 

suppose  a  line  500  miles  long  ordinarily  measures  4500  ohms,  or  9  ohms 
per  mile,  and  the  resistance  measured  through  a  dead  ground  is  1800 
ohms,  then  the  ground  is  200  miles  from  the  station  where  the  measure- 
ment is  made,  since  9  times  200  is  equal  to  1800. 

When  the  ground  is  only  partial,  its  location  is  not  so  simple,  since 
the  resistance  of  the  leakage  path  comes  into  the  measurements.  Sev- 
eral methods  may  be  used  in  making  the  measurements,  but  the  two 
following  are  the  simplest.  In  the  first  method,  the  resistance  of  the  line 
through  the  Fault  is  measured  from  each  end  in  the  manner  illustrated 
in  Figure  311,  the  far  end  being  open  at  the  time  of  each  measurement. 
To  find  the  resistance  of  the  line  between  one  end  E,  and  the  fault, 
the  resistance  of  the  line  in  good  order  is  added  to  that  measured 
through  the  fault  from  E  ;  from  this  is  subtracted  the  resistance  meas- 
ured through  the  fault  from  G  and  the  result  is  divided  by  2.  For 
instance,  suppose  the  resistance  measured  through  the  fault  from  E,  as 
shown  in  Figure  311,  is  3800  ohms,  and  a  similar  measurement  made 

1  Article  92. 


416 


ELECTRICITY   AND   MAGNETISM 


from  G  shows  4700  ohms,  the  line  itself  from  E  to  G  measures  (as  known 
by  previous  measurements)  4500  ohms ;  then  the  resistance  of  the  line 
from  E  to  the  fault  is 

3800  -j-  4500  —  4700 
2 


= 1800. 


If  the  line  measures  9  ohms  to  the  mile,  the  distance  from  E  to  the 
fault  is  200  miles. 

The  reason  for  this  is  readily  seen,  since  the  total  resistance  of  the 
line  is  equal  to  the  resistance  from  E  to  the  fault,  added  to  that  from 
G  to  the  fault.  The  measurement  from  E  through  the  fault  gives  the 
resistance  from  E  to  F  added  to  the  resistance  of  the  leak.  The  meas- 
urement from  G  through  the  fault  gives  the  resistance  from  G  to  F 
added  to  the  resistance  of  the  leak.  Adding  together  the  resistance  of 
the  whole  line  in  good  order  and  the  resistance  from  E  through  the 
fault,  gives  a  sum  which  is  equal  to  the  resistance  of  the  leak  plus  the 
resistance  of  the  whole  line  plus  the  resistance  of  the  part  of  the  line 
from  E  to  F.  Subtracting  the  resistance  from  G  through  the  leak  leaves 
a  remainder  equal  to  twice  the  resistance  of  the  line  from  E  to  F. 

Example  A.  The  resistance  of  the  portion  of  a  line  from  the  end  E  (Fig.  311)  to 
a  partial  ground  is  2000  ohms,  from  the  end  G  is  4000  ohms,  and  the  total  resistance 
of  the  line  is  3800  ohms.  If  the  wire  has  a  resistance  of  10  ohms  to  the  mile,  what 
is  the  distance  from  the  end  E  to  the  partial  ground?  Ans.  90  miles. 

358.  Loop  Method  for  locating  Fault.  — The  second  method  of  locat- 
ing a  fault  is  by  what  is  called  the  Loop  Method.  This  can  be  used  only 

when  the  leaky  wire  can  be  looped  with  a 
good  wire  or  the  line  is  a  metallic  circuit, 
so  that  both  ends  may  be  connected  to 
a  bridge  for  testing.  In  this  case  the  con- 
nections are  made  up  as  shown  in  diagram 
in  Figure  312,  where  DE  is  the  leaky  wire 
and  CE  is  the  good  one.  AF  makes  one 
bridge  arm,  and  CF  another,  while  AB 
and  BC  are  the  other  two  arms.  When  AD 
or  AB  and  BC  are  adjusted  until  the  bridge 
is  balanced,  the  resistance  from  C  to  F  and  from  A  to  F  are  to  each 
other  as  BC  is  to  AB,  while  the  total  resistance  of  CE  plus  DE  plus 


GROUND 

FIG.  312.  —  Diagram  of  Connec- 
tions for  locating  a  "Fault" 
by  Loop  Method. 


LINE  TESTING 


417 


AD  are  known  from  the  records  of  the  wire  conductivities  and  the  read- 
ing of  the  rheostat  AD.     The  way  in  which  the  connections  are  made 
to  a  post-office  pattern  bridge  is  shown 
in  Figure  313. 


Example  A.  In  Figure  312,  AB=$oo  ohms; 
BC  —  1000  ohms;  AD  =  200  ohms;  CE  =  900 
ohms;  and  DE  =  400  ohms.  If  under  these 
conditions  the  bridge  is  balanced,  how  far  from 
D  is  the  break,  supposing  the  resistance  of  the 
line  is  5  ohms  per  mile?  (Aid.  By  the  pro- 
portion of  the  bridge  we  have  AB  \BC\\  AD 
+  DE  -  EF  :  CE  +  EE;  from  this  it  is  found 
that  EF—  100  ohms.)  Ans.  60  miles.  • 


GROUND 

FIG.  313.  —  Post-office  Pattern  Bridge 
applied  to  Loop  Test  for  locat- 
ing a  Fault  in  a  Telegraph  or 
Telephone  Line. 


359.  Location  of  a  Cross. — When  two 
wires  are  crossed,  the  location  of  the 
point  where  they  make  contact  with 

each  other  is  carried  out  in  very  much  the  same  manner  as  the  location 
of  grounds,  except  that  the  measurements  are  made  over  a  circuit  made 
up  of  the  two  crossed  wires  (Fig.  314)  instead  of  over  a  circuit  made 
up  of  the  grounded  wire  and  its  ground  return.  The  distance  from 

the  measuring  station  to  the 

| 1  cross  is  calculated  from  the 

measured  resistance  and  the 
resistance  per  mile  of  the 
two  wires  together.  Thus, 
•suppose  the  resistance  meas- 

FlG.  314.  —  Diagram  of  Connections  /or  locatyjg'a  ,  . 

Cross  in  Telegraph  or  Telephone  Lines.  Ured  th/OUgh  the  crOSS  at  Z 

as'^fiown  in  Figure  314  is 

4400  ohms,  and  the  resistances  of  the  two  wires  are  respectively  9  and 
13  ohms  per  mile.  Then  the  cross  is  200  miles  from  the  measuring 
station,  since  the  resistance  per  mile  of  the  two  wires  together  is  9  plus 
13,  or  22  ohms.  In  this  measurement  it  is  assumed  that  the  resistance 
at  the  cross  itself  is  too  small  to  be  taken  into  account.  When  this  is 
not  the  case,  special  measurements  have  to  be  made,  as  in  the  case  of  a 
partial  ground. 

In  making  test  measurements  it  is  usual  to  disconnect  all  telegraph  or 


4i8 


ELECTRICITY   AND   MAGNETISM 


FiG.  315.  —  Table  with   Instruments  permanently 
set  up  for  Daily  Use  at  Ocean  Cable  Station. 


telephone  instruments  from  the  circuit,  though  they  may  be  permitted 
to  remain  in  circuit  and  a  correction  is  then  made  on  account  of  their 

resistance  or  insulation. 

360.  Testing  Underground 
Cables.  —  In  testing  under- 
ground wires  and  Submarine 
Cables,  practically  the  same 
methods  are  used  as  in  the 
testing  of  overhead  wires. 
Systematic  periodical  tests 
are  quite  essential  for  the 
preservation  of  the  life  of 
cables,  since  their  usefulness 
may  be  quickly  destroyed 
after  a  leak  starts.  Figure 
315  shows  the  permanent 

testing  arrangements  as  they  are  set  up  in  the  testing  room  at  the  end 
of  an  ocean  cable. 

361.  Faults  in  Electric  Light  and  Power  Lines.  —  The  faults  which 
occur  in  electric  light  and  power  lines  are  of  the  snme  character  as  those 
which  occur  in  telegraph  and  telephone  lines,  but  the  methods  of  test- 
ing for  and  locating  the  faults  are  very  different.  The  general  condition 
of  an  electric  light  line  may  be  determined  from  the  manner  in  which 
the  lights  burn.  Breaks  in  the  line  are  made  evident  by  the  fact  that 
lamps  on  the  circuit  beyond  the  break  will  not  burn,  while  crosses  and 
short  circuits  soon  make  themselves  evident  by  causing  the  fuses  which 
protect  the  defective  part  of  the  circuit  to  melt  or  blow.  Poor  connec- 
tions may  be  shown  by  dimness  of  the  lamps  when  the  connections  have 
a  sufficiently  high  resistance  to  cause  a  great  drop  in  pressure.  It  is 
needless  to  say  that  connections  or  joints  of  such  poor  conductivity  are 
very  dangerous,  and  should  not  be  permitted  to  exist  in  a  circuit  for  an 
hour.  All  joints  in  electric  light  wires  are  soldered  in  order  that  there 
may  be  no  "  bad  joints  "  which  may  cause  poor  connections. 

Poor  connections  at  such  points  as  sockets  or  fuse  blocks  belonging 
to  incandescent  circuits  may  cause  considerable  heating.  If  such  heat- 
ing is  noticed,  it  should  be  corrected  at  once,  or  it  may  cause  damage. 


LINE  TESTING 


419 


Sometimes  poor  connections  at  fuse  blocks  may  produce  heat  enough  to 
cause  the  fuses  to  blow  when  there  is  really  no  trouble  elsewhere  on  the 
circuit.  This  may  occur  when  the  fuse  blocks  have  too  little  contact 
surface  at  the  connection  points  to  properly  carry  the  current.  Such 
fuse  blocks  should  always  be  replaced  by  larger  and  better  ones,  as  they 
are  not  only  an  annoyance,  but  are  dangerous. 

No  one  would  think  for  a  moment  of  leaving  poorly  jointed  and 
leaky  gas  pipes  and  fixtures  in  a  house,  and  defective  electric  wires 
should  be  treated  in  exactly  the  same  manner  as  defective  gas  fittings. 

362.  Arc-line  Testing  —  The  Magneto.  —  Series  circuits,  like  arc- 
light  circuits,  which  are  not  in  use  all  through  the  twenty-four  hours,  are 
often  tested  for  breaks,  grounds,  and  crosses  by  means  of  a  "  magneto 
bell,"  which  is  very  much  like  a  telephone  call  bell  (Fig.  316).  The 
little  magneto  machine  and  call  bell  are  put  in  a  box  together  and  con- 
nected in  series  with  two  terminals  on  the 
outside  of  the  box,  which  are  shown  at  the 
top  of  the  figure.  If  it  is  desired  to  test 
the  Continuity  of  a  line,  —  that  is,  the 
absence  of  breaks,  —  the  two  ends  of  the 
line  are  connected  to  the  test  bell  termi- 
nals. If  the  bell  rings  when  the  crank, 
which  is  on  the  right-hand  side  of  the  box 
(but  hidden  in  the  figure),  is  turned,  the 
circuit  is  shown  to  be  continuous ;  while  if 
the  bell  does  not  ring,  the  circuit  is  shown 
to  be  broken,  provided  the  test  bell  itself 

is  in  good  condition.  It  is  easy  to  test  the  latter  by  short-circuiting  the 
terminals,  when  the  bell  will  ring  upon  turning  the  crank  if  the  magneto 
is  all  right. 

If  it  is  desired  to  test  for  grounds  by  means  of  a  magneto  bell,  one 
terminal  of  the  bell  is  connected  to  earth  by  connecting  it  through  a 
wire  to  a  gas  or  water  pipe,  and  the  other  terminal  is  connected  to  the 
line  to  .be  tested.  If  the  bell  rings  when  the  crank  is  turned,  this 
ordinarily  means  that  the  line  is  grounded,  and  if  the  bell  does  not  ring, 
the  line  is  shown  to  be  Clear  of  grounds.  Sometimes  the  bell  will  ring 
a  little  when  the  line  has  a  very  high  insulation,  because  the  electrostatic 


FIG.   316.  —  Portable  Magneto 
and  Bell  for  Use  in  Testing. 


420 


ELECTRICITY   AND    MAGNETISM 


capacity  of  the  line  is  high,  and  the  current  which  flows  into  and  out  of 
the  line,  as  it  is  charged  and  discharged  by  the  alternating  pressure  set 
up  by  the  magneto,  is  sufficient  to  ring  the  bell. 

Most  arc-light  lines  are  out  of  use  during  daylight,  —  only  those  which 
convey  current  to  arc  lamps  in  the  buildings  of  large  cities  are  used 
during  the  day,  —  and  many  lines  are  not  used  after  midnight.  It  is 
quite  convenient,  therefore,  to  use  the  magneto  bell  for  testing  such 
lines.  The  tests  can  be  made  an  hour  or  two  before  the  lines  come 
into  service  each  day,  and  if  anything  is  found  to  be  wrong,  a  lineman 
can  go  along  the  line  to  find  the  trouble  and  fix  it. 

363.  Testing  Arc  Circuits  by  Voltmeter  or  Incandescent  Lamps.  —  A 
voltmeter  is  sometimes  used  for  testing  and  locating  grounds  on  arc- 
light  lines  while  they  are  in  use.  Sup- 
pose that  Figure  317  represents  an  arc- 
light  line  which  supplies  current  to  n 
lamps,  and  is  grounded  at  F.  If  the 

|~®~  I    ((Sij  lamps  are  so  adjusted  that  each  requires 

JL      l__r^%__  45  volts  pressure,  the  difference  of  press- 

ure between  the  fault  and  one  terminal 
of  the  dynamo  is  2  70  volts,  and  between 
the  fault  and  the  other  terminal  of  the 
dynamo  the  difference  of  pressure  is  225 
volts.  A  voltmeter  connected  to  ground, 


AMPERE 
METER 


A 

VOLT  V- 
METER 


GROUND 


FIG.  317.  —  Testing  the  Insulation 
of  an  Arc-light  Circuit  by  a 
Voltmeter. 


as  shown   in    Figure    317,    indicates    the 

difference  in  pressure  between  the  fault  and  one  dynamo  terminal,  and 
so  shows  between  what  lamps  the  fault  is  located. 

Instead  of  using  a  voltmeter,  45 -volt  incandescent  lamps  may  be 
used  for  testing  by  this  method.  As  many  45 -volt  incandescent  lamps 
are  connected  in  series  as  there  are  arc  lamps  on  the  circuit  to  be 
tested.  One  end  of  the  series  is  connected  to  ground,  and  the  other  to 
one  dynamo  terminal.  Then  one  incandescent  lamp  after  another  is 
short-circuited  until  the  lamps  which  remain  in  the  circuit  burn  to  their 
full  candle  power.  The  number  of  incandescent  lamps  then  in  circuit  is 
equal  to  the  number  of  arc  lamps  between  the  dynamo  terminal  and  the 
fault.  The  reason  for  this  is  evident  upon  examining  the  figure.  Since 
there  are  six  arc  lamps  between  the  A  terminal  of  the  dynamo  and  the 


LINE  TESTING 


421 


FIELD 

RESISTANCE 
BOX 


fault,  there  is  a  difference  of  pressure  of  270  volts  between  the  two 
points,  as  shown  by  the  voltmeter.  Two  hundred  and  seventy  volts  is 
the  pressure  required  to  bring  a  series  of  six  45 -volt  incandescent 
lamps  to  full  candle  power,  so  that  the  number  of  arc  lamps  between 
the  fault  and  the  A  dynamo  terminal  is  equal  to  the  number  of  45-volt 
incandescent  lamps  which  will  burn  with  full  candle  power  when  con- 
nected in  series  between  the  dynamo  terminal  and  the  ground.  This  test 
is  made  upon  the  supposition  that  the  fault  has  little  resistance  in  itself. 

Example  A.  A  circuit  containing  10  arc  lamps,  each  adjusted  to  burn  with  a 
pressure  of  50  volts,  is  so  grounded  that  when  a  voltmeter  is  connected  between 
one  terminal  and  the  ground  the  reading  is  200  volts.  Between  which  two  larrips  is 
the  location  of  the  ground,  counting  from  the  terminal  to  which  the  voltmeter  is 
connected?  Ans.  Between  the  fourth  and  fifth. 

364.    Testing  Constant  Pressure  Circuits  —  The  Ground  Detector. — 

In  testing  incandescent  lighting  circuits  for  grounds,  incandescent  lamps 
or  voltmeters  are  almost  always  used. 
If  one  wire  of  a  two-wire  circuit  is 
grounded,  the  presence  of  the  ground 
may  be  shown  by  connecting  an  in- 
candescent lamp  between  the  other 
wire  and  the  earth  (water  pipes,  etc., 
Fig.  318),  when  the  lamp  will  burn 
on  account  of  the  current  which  flows 
from  one  wire  to  the  other  through 
the  lamp  and  the  fault.  If  the  lamp 
is  intended  for  the  same  pressure 
as  that  of  the  circuit,  it  will  burn  at 
full  candle  power  if  the  circuit  is 

"  dead  grounded,"  and  will  be  dimmer  in  proportion  to  the  resistance 
of  the  fault. 

Figure  319  shows  a  permanent  arrangement  of  the  Ground  Detector, 
which  is  fixed  so  that  the  detector  lamp  may  be  connected  at  pleasure 
with  either  of  the  wires. 

Another  arrangement  of  lamps  for  a  ground  detector  is  shown  in  Fig- 
ure 320,  where  A  and  B  are  two  lamps  connected  in  series  between  the 
two  wires  of  the  electric  lighting  system.  A  wire  goes  to  ground  through 


FIG.  318.  —  Incandescent  Lamp  used  as  a 
"  Ground  Detector." 


422 


ELECTRICITY   AND    MAGNETISM 


WOUND 

FIG.  319.  —  Incandescent  Lamp 
used  as  a  "  Ground  Detec- 
tor." 


a  fuse  block  and  a  switch  from  a  point  between  the  two  lamps.  When 
the  switch  is  open,  the  lamps  A  and  B  burn  very  dimly,  but  of  equal 

brightness,  and  no  change  occurs  when  the 
switch  is  closed  if  no  grounds  are  present 
on  the  circuit.  But  if  the  wire  to  which 
the  A  lamp  is  connected  becomes  grounded, 
current  will  flow  from  the  grounded  wire 
through  the  B  lamp  to  the  other  wire  when 
the  switch  is  closed,  and  the  B  lamp  will 
become  brighter  than  the  A  lamp.  In  the 
same  way  the  A  lamp  will  brighten  when  the 
switch  is  closed,  if  the  B  wire  is  grounded. 
Sometimes  both  wires  are  grounded,  and 

the  faults  have  about  equal  resistance.  In  this  case  the  lamps  will  not 
show  the  grounds  in  the  ordinary  way,  but  the  test  can  be  made  by 
turning  off  one  lamp  when  the  switch  is 
closed,  and  the  other  lamp  will  go  out 
if  the  circuit  is  not  grounded. 

When  high  pressures  are  used,  too 
many  lamps  would  be  required,  so  that 
a  differential  galvanometer  or  equivalent 
device  is  used  as  a  ground  detector. 
The  differential  galvanometer  consists 
essentially  of  two  equal  coils,  one  over 
the  other,  which  surround  a  suspended 
magnetic  needle.  The  coils  are  con- 
nected to  the  circuit  in  series,  and  with 

a  ground  connection  between  them  just  as  the  lamps  are  connected,  but 
their  magnetic  effects  on  the  needle  are  in  opposition.  When  there  is 
no  ground,  the  needle  stands  at  zero ;  but  when  there  is  a  ground,  the 
magnetic  effect  of  one  of  the  coils  becomes  stronger,  and  the  needle  is 
deflected.  For  very  high  pressures,  electrostatic  ground  detectors  are 
frequently  used.  An  electrostatic  ground  detector  is  illustrated  in  Fig- 
ure 321.  It  consists  essentially  of  a  quadrant  electrometer  with  one 
quadrant  connected  to  each  side  of  the  circuit,  and  the  other  two  con- 
nected to  the  needle  and  to  ground.  The  needle  stands  at  zero  when 


SAFETY  FUSE 


GROUND 

FlG.  320.  —  Ground  Detector  with 
Two  Incandescent  Lamps  and 
Ground  Connection  between 
them. 


LINE  TESTING 


423 


neither  side  of  the  circuit  is  grounded,  but  moves  to  one  side  or  the 
other  if  one  wire  or  the  other  becomes  grounded. 

For  three-wire  circuits1  a  pair  of  lamps  may  be  used  as  a  ground 
detector  for  each  side  of  the  system. 

When  a  voltmeter  is  used  to  test  for  grounds  on  an  incandescent  cir- 
cuit, it  is  employed  in  very  much 
the  same  way  as  the  incandescent 
lamp  which  is  used  for  the  same 
purpose.  The  voltmeter  is  con- 
nected between  one  of  the  circuit 
wires  and  the  earth.  If  the  other 
wire  of  the  circuit  is  grounded, 
current  will  flow  from  it  through 
the  ground  to  the  voltmeter,  and 
through  it  to  the  wire  to  which 
the  instrument  is  connected. 
If  the  grounded  wire  is  "  dead 
grounded,"  the  voltmeter  will 
give  the  same  reading  as  when  it 
is  connected  directly  between  the 
wires.  The  reading  of  the  volt- 
meter is  less  as  the  resistance  of  the  ground  contact  is  greater,  and 
it  is  zero  when  the  insulation  is  perfect. 

365.  Locating  Grounds  on  Electric  Light  Wires.  —  The  methods  which 
are  used  for  testing  for  grounds  on  incandescent  circuits  show  when 
a  ground  is  present  and  upon  which  wire  it  exists,  but  they  do  not  give 
any  clew  to  the  particular  portion  of  the  circuit  upon  which  the  ground 
is  located.  The  ordinary  method  of  "  locating  "  a  ground  which  cannot 
be  found  by  inspection  is  to  cut  one  branch  after  another  off  from  the 
system  until  the  ground  disappears.  The  ground  is  then  on  the  last 
branch  cut  off,  and  may  be  found  by  careful  inspection.  If  the  wires  are 
not  being  used,  the  testing  of  the  branches  may  be  done  by  the  use  of 
a  magneto,  as  in  arc  lighting.  When  buildings  are  newly  wired,  all  lines 
should  be  carefully  tested  by  a  magneto  for  breaks,  crosses,  and  grounds, 


[GROUND 
FIG.  321. —  Electrostatic  Ground  Detector. 


Article  334. 


424  ELECTRICITY   AND    MAGNETISM 

and  then  careful  insulation  measurements  with  instruments  should  be 
made  before  closing  the  dynamo  switches. 

The  testing  and  locating  of  faults  in  lead-covered  cables  which  are 
sometimes  used  in  underground  systems  is  done  in  the  same  way  as  the 
insulation  testing  of  telegraph  and  telephone  cables,  which  has  already 
been  explained.1 

366.  Measurement  of  Candle  Power.  —  The  candle  power  and  the  best 
arrangement  of  the  lamps  which  are  required  to  give  a  satisfactory  illu- 
mination in  any  particular  space  can  be  determined  only  by  experience. 
The  candle  power  of  lamps  is  measured  by  an  instrument  called  a  pho- 
tometer, in  which  the  illuminating  power  of  the  lamp  to  be  measured  is 
directly  compared  with  the  power  of  Standard  Candles,  or  with  a  gas  jet 
or  lamp  of  known  candle  power.  Standard  candles  are  made  of  sperm 
wax ;  their  wick  is  a  very  carefully  made  cotton  braid  ;  and  candles  of 
full  length  (ten  inches)  weigh  one-sixth  of  a  pound  apiece.  Each  can- 
dle should  burn  with  a  consumption  of  wax  at  the  rate  of  120  grains 
consumed  per  hour.  These  candles  should  not  usually  be  snuffed.  In 
Germany,  candles  of  somewhat  different  character  are  used. 

The  commonest  form  of  a  photometer  is  that  called  Bunsen's  photom- 
eter, which  is  shown  diagrammatically  in  Figure  322.  The  standard 

candle  is  shown  at  A,  the 
lamp  whose  illumination  is 
to  be  measured  is  at  B,  and 
D  is  a  movable  disk  of  thin 
paper  with  a  grease  spot  at 
its  centre.  The  photometer 

FIG.  322.- Simple  Bunsen  Photometer.  must  be   enclosed   in   a  per- 

fectly dark  closet  for  satis- 
factory use,  and  the  light  from  A  and  B  is  carefully  screened  on  every 
side  except  directly  in  line  with  the  disk.  An  observer  measures  the 
unknown  candle  power  of  the  lamp  B  by  moving  the  disk  D  until 
it  shows  an  equal  illumination  on  both  sides.  The  disk  is  generally 
looked  at  by  means  of  mirrors,  so  that  both  sides  may  be  seen  at 
once. 

When  the  illumination  of  the  two  sides  of  the  disk  is  equal,  the  candle 
1  Articles  356  to  359. 


MEASUREMENTS   OF  ILLUMINATION  425 

powers  of  the  lights  are  related  to  each  other  in  the  ratio  of  the  squares 
of  the  distances  measured  from  the  respective  lights  to  the  disk. 

Example  A.  A  in  Figure  322  is  a  standard  candle,  and  B  an  electric  light,  the 
candle  power  of  which  is  to  be  measured.  If  the  distance  from  A  to  the  screen  is  20 
inches,  and  from  B  to  the  screen  is  80  inches,  what  is  the  candle  power  of  the  electric 
light  ?  Ans.  1 6  c.p. 

367.  Light  varies   inversely  as  the  Square  of  the  Distance. — The 

reason  that  the  squares  of  the  distances  come  into  the  comparison  of 

candle  powers  is  illustrated  in  Figure  323.    If  we  suppose  a  screen,  AB, 

to  be  placed  at  a  distance  of  one  foot  from  the  lamp,  Z,  we  may  consider 

that  the  screen  is  illuminated  by  a  certain 

number   of  rays    of  light    falling   upon   it. 

Now,  if  the  screen  is  moved  to  a  distance 

of  two  feet  from  the  lamp,  the  same  rays 

of  light  will  illuminate  an  area  CD,  which 

is  four  times  as  .arge  as  A3,  and  conse- 

quently  the  intensity  of  the  illumination  of          inverse  Proportion  with  the 

the  screen  is  only  one-fourth  as  great  as         Square  of  the  Distance  from 

when  the  screen  was  at  a  distance  of  one 

foot  from  the  lamp.     If  the  screen  is  moved  to  a  point  three  feet  from 

the  lamp,  the  same  rays  will  cover  the  area  EF,  which  is  nine  times 

as  large  as  AB,  and  the  intensity  of  the  illumination  is  only  one-ninth 

as  great  as  when  the  screen  was  within  a  foot  of  the  lamp. 

Since  four  and  nine  are  respectively  equal  to  the  squares  of  two  and 
three,  we  see  that  the  intensity  of  the  illumination  given  to  a  surface  by 
a  fixed  light  is  inversely  proportional  to  the  square  of  the  distance  from 
the  light  to  the  surface. 

In  the  Bunsen  photometer  the  screen  is  placed  at  such  a  point 
directly  between  two  lights  that  they  illuminate  it  equally.  In  this 
case  the  lights  must  have  candle  powers  which  are  proportional  to  the 
squares  of  their  distances  from  the  screen,  as  already  said. 

368.  Distribution  of  Lights. — The  actual   illuminating  effect  of  a 
given  number  of  lamps  in  any  space  depends  upon  a  great  many  things. 
For  instance,  a  room  with  dark  walls,  which  absorb  a  great  deal  of  light, 
requires  much  more  light  to  give  a  satisfactory  illumination  than  does  a 
room  with  light-colored  or  white  walls.     In  a  comparatively  small  space 


426  ELECTRICITY   AND   MAGNETISM 

a  number  of  lamps  of  small  candle  power,  properly  distributed  about  the 
space,  usually  give  a  more  satisfactory  light  than  do  a  few  large  lamps 
giving  the  same  total  candle  power.  This  is  because  the  illumination  near 
the  large  lamps  is  very  great,  and  at  other  points  in  the  space  the  illumina- 
tion is  small,  while  it  is  much  more  evenly  distributed  by  the  small  lamps. 

In  ordinary  rooms  and  stores  it  is  common  to  put  from  one  to  three 
i6-candle-power  incandescent  lights  for  each  hundred  square  feet  of 
floor,  while  in  larger  rooms  45o-watt  enclosed  arc  lamps  may  be  used 
so  that  each  arc  illuminates  from  500  to  1000  square  feet  of  floor 
space.  The  incandescent  lights  should  be  suspended  in  such  a  way 
as  not  to  be  more  than  eight  feet  from  the  floor,  and  be  provided 
with  reflectors  or  shades.  If  the  lamps  are  placed  higher,  proportion- 
ately more  lights  must  be  used. 

Where  arcs  are  placed  incToors,  it  is  usual  to  surround  the  arc  with  an 
opal  glass  globe,  which  distributes  the  light  more  satisfactorily  than 
would  otherwise  be  the  case.  Such  globes  have  the  disadvantage  of 
absorbing  nearly  one-half  of  the  light  of  the  arc,  but  their  effect  in  dis- 
tributing the  light  is  sufficiently  important  in  indoor  lighting  to  counter- 
balance the  loss  of  light.  Arc  lights  should  be  placed  higher  from 
the  floor  than  is  usual  for  incandescents.  For  outdoor  lighting  open 
arc  lights  with  clear  glass  globes  have  been  ordinarily  used,  but 
"  enclosed "  arc  lamps,  with  opalescent  inner  globes  and  clear  glass 
outer  globes,  are  now  rapidly  coming  into  general  use. 

Street  lamps  (arcs)  are  placed  from  50  to  600  feet  apart,  depending 
upon  the  amount  of  illumination  desired.  It  is  an  important  fact, 
which  is  not  very  well  known  by  electric  light  companies,  that  dirty 
globes  of  clear  glass  may  absorb  even  more  light  than  do  opal  globes, 
and  in  the  case  of  the  inner  globes  of  the  enclosed  lights  it  is  not  unusual 
for  the  light  to  be  reduced  to  a  mere  glow  by  a  little  inattention,  so  that 
it  is  important  that  arc-light  globes  be  kept  clean.  If  the  "  enclosed 
arc  "  lamps  are  carelessly  trimmed,  carbon  dust  rapidly  collects  and  is 
burned  into  the  glass,  which  often  makes  the  globe  almost  opaque. 

369.  Measure  of  Illumination  —  The  Candle  Foot.  —  It  may  be  seen 
from  what  precedes  that  the  true  measure  of  illumination  is  not  the 
candle  power  of  a  lamp,  but  it  is  the  amount  of  light  or  Illumination 
obtained  on  a  surface  which  is  illuminated  by  the  lamp.  The  unit  for 


MEASUREMENTS  OF  ILLUMINATION  427 

measuring  illumination  is  the  intensity  of  illumination  on  a  perpendicular 
screen  at  the  distance  of  one  foot  from  a  lamp  which  gives  one  candle  power. 

Four  candle  power  at  a  distance  of  two  feet  from  the  screen,  and  nine 
candle  power  at  a  distance  of  three  feet  from  the  screen,  give  the  same 
illumination  as  one  candle  power  at  a  distance  of  one  foot  from  the 
screen.  This  illumination  is  called  a  Candle  Foot. 

The  illumination  (in  candle  feet}  given  by  any  lamp  upon  a  perpen- 
dicular surface  is  equal  to  the  candle  power  of  the  lamp  divided  by  the 
square  of  the  distance  between  the  lamp  and  the  surface.  For  instance, 
if  we  have  a  3  2 -candle-power  lamp  at  a  distance  of  6  feet  from  a  wall, 
the  illumination  on  the  wall  is  32  divided  by  6  squared  (or  36),  which 
is  equal  to  about  .9  of  a  candle  foot  (ff  =  .889). 

Example  A.  An  incandescent  lamp  is  4  feet  from  a  reading  book.  How  much 
illumination  will  it  give  for  reading,  if  the  lamp  gives  16  candle  power  in  the  direc- 
tion of  the  book?  Ans.  I  candle  foot. 

Example  B.  An  arc  lamp  gives  800  candle  power  in  a  certain  direction.  What 
will  be  the  illumination  from  it  at  a  distance  of  200  feet  in  that  direction?  Ans.  -fa 
of  a  candle  foot. 

An  illumination  of  one  candle  foot  is  rather  poor  for  ordinary  reading, 
but  an  illumination  equal  to  from  two  to  three  candle  feet  is  very  satis- 
factory, especially  if  the  direct  view  of  the  lights  does  not  meet  the  eyes 
and  so  reduce  their  sensitiveness.  Ordinary  bright  moonlight  gives  an 
illumination  on  the  ground  which  is  equal  to  about  ^%-Q  of  a  candle 
foot.  The  illumination  upon  theatre  stages  is  ordinarily  from  three  to 
four  candle  feet,  and  the  illumination  given  by  diffused  daylight  is  equal 
to  from  ten  to  forty  candle  feet.  On  account  of  the  expense  of  produc- 
ing artificial  light  by  the  common  methods  of  the  present  day,  it  is 
commercially  impracticable  to  artificially  produce  as  great  an  illumina- 
tion as  may  be  given  by  daylight. 

A  handy  plan  for  roughly  comparing  the  illumination  in  various 
situations  is  the  following :  A  cubical  box,  six  or  eight  inches  on  a  side, 
and  without  a  cover,  should  be  blackened  on  the  inside  and  a  piece  of 
newspaper  placed  in  its  bottom.  The  ease  with  which  the  words  on  the 
paper  may  be  read  when  looking  into  the  box  with  shaded  eyes  gives  a 
rough  and  ready  determination  of  the  quality  of  the  illumination  at  the 
spot  occupied  by  the  box.  By  moving  around  a  room  with  the  box,  a 


428 


ELECTRICITY  AND   MAGNETISM 


rough  determination  of  the  uniformity  of  the  illumination  in  the  room,  and 
therefore  of  the  correctness  of  the  locations  of  the  individual  lights,  may 
be  made.  By  taking  the  box  from  room  to  room  or  building  to  building, 
a  rough  comparison  may  be  made  of  the  illumination  at  different  places. 
A  special  photometer,  by  means  of  which  such  comparisons  may  be  made 
with  great  accuracy,  is  called  a  Weber  photometer.  The  actual  illumina- 
tion at  any  point  may  be  directly  measured  in  candle  feet  by  means  of  this 

instrument.  Figure  324 
shows  a  Weber  photometer 
ready  to  measure  the  illu- 
mination on  a  library  read- 
ing table. 

It  must  be  remembered 
that  it  is  not  always  desir- 
able to  have  a  perfectly 
uniform  illumination  in  a 
room,  but  is  often  neces- 
sary to  have  a  high  degree 
of  illumination  at  certain 
points.  In  library  reading 
rooms,  for  instance,  the 
reading  tables  should  be 
most  highly  illuminated. 
In  picture  galleries,  the 
pictures  should  have  a 
strong  light  thrown  on 
them.  In  theatres  it  should  be  possible  to  vary  the  lights,  and  also  be  pos- 
sible to  throw  an  intense  illumination  upon  any  part  of  the  stage.  On  the 
other  hand,  schoolrooms,  drawing-rooms,  ballrooms,  and  other  like  rooms 
should  ordinarily  be  provided  with  illumination  that  is  as  nearly  uniform 

as  possible. 

QUESTIONS 

1.  What  are  the  classes  of  trouble  that  occur  on  telegraph  and  telephone  lines? 

2.  What  is  a  ground?     A  cross? 

3.  How  can  the  position  of  trouble  on  a  telegraph  line  be  located? 

4.  How  does  trouble  usually  become  apparent  on  telephone  lines? 

5.  What  is  cross  talk?     How  can  it  be  avoided? 

6.  How  is  the  conductivity  of  a  long  line  measured? 


n  *  C";  A'.  1% 

FIG.  324.  —  Weber  Photometer  standing  on   Reading 
Room  Table  in  Library  of  University  of  Wisconsin. 


MEASUREMENTS  OF  ILLUMINATION  429 

7.  How  may  the  conductivity  of  three  parallel  wires  be  measured? 

8.  What  are  earth  currents? 

9.  How  may  the  insulation  of  a  long  line  be  measured  ? 

10.  How  can  a  "  dead  ground  "  be  located? 

11.  How  can  a  partial  ground  be  located  by  taking  measurements  from  either 
end  of  the  line? 

12.  What  is  the  loop  method  of  rinding  a  ground? 

13.  How  can  a  cross  be  located  ? 

14.  WThat  faults  occur  in  electric  light  circuits? 

15.  How  do  faults  in  electric  light  circuits  make  themselves  evident? 

1 6.  Why  does  safety  require  that  the  joints  in  electric  light  wires  shall  be  soldered  ? 

17.  What  is  a  testing  magneto? 

1 8.  How  can  arc  circuits  be  tested  for  continuity  and  grounds  by  a  magneto? 

19.  How  may  a  fault  be  located  on  an  arc  circuit  by  a  voltmeter?     By  incan- 
descent lamps? 

20.  What  is  a  ground  detector  ?     On  what  kinds  of  circuits  is  it  used  ? 

21.  How  does  a  ground  detector  with  one  lamp  work? 

22.  Why  does  one  of  the  lamps  brighten  in  a  two-lamp  ground  detector  when 
there  is  a  ground  on  one  wire? 

23.  How  may  a  voltmeter  be  used  for  testing  for  grounds  on  incandescent  circuits? 

24.  If  one  wire  of  a  circuit  is  dead  grounded,  what  will  be  the  readings  of  the 
voltmeter  when  connected  from  the  ground,  first  to  the  grounded  wire,  then  to  the 
other  wire? 

25.  How  are  grounds  or  crosses  located  on  constant  potential  circuits? 

26.  Can  a  magneto  be  used  in  testing  constant  potential  circuits? 

27.  Tell  how  you  would  test  for  continuity  and  grounds  in  a  building  that  had 
just  been  wired  for  incandescent  lamps.     How  would  you  test  for  insulation  resistance  ? 

28.  What  is  a  photometer? 

29.  What  is  a  standard  candle  ? 

30.  How  may  the  intensity  of  two  lights  be  compared  by  a  photometer? 

31.  Why  does  the  intensity  of  illumination  vary  inversely  as  the  square  of  the 
distance  from  the  light? 

32.  What  is  the  effect  of  the  color  of  the  walls  and  the  size  of  the  light  upon  the 
distribution  in  a  room? 

33.  What  is  the  effect  of  opal  shades  or  globes  upon  the  light  from  a  lamp  ?     Of 
dirty  globes? 

34.  \Vhat  is  the  candle  foot  ? 

35.  How  does  the  intensity  of  illumination  in  candle  feet  vary  with  the  distance 
from  the  light? 

36.  What  intensity  of  illumination  is  desirable  for  reading? 

37.  WThat  is  the  intensity  of  illumination  of  moonlight?     Of  sunlight? 

38.  How  can  you  roughly  compare  the  illumination  in  various  parts  of  a  room? 


CHAPTER    XXII 

ELECTROLYTIC   DEPOSITION   OF   METALS 
ELECTRIC   SiMELTING,    WELDING,  COOKING,  ETC. 

370.  Commercial  Electrolysis.  —  The  electrochemical  operations 
which  result  in  depositing  metals  from  solutions  of  their  metallic 
salts  are  very  widespread  in  the  industries,  and  are  of  great  usefulness. 
The  magnitude  of  the  works  involved  in  most  of  the  operations  does  not 
approach  that  of  works  built  for  the  purpose  of  furnishing  electricity  for 
light  and  power ;  nor  do  the  ordinary  electrolytic  operations  appeal  to 
the  ordinary  observer  as  do  the  applications  of  electricity  to -transmitting 
messages,  driving  street  cars,  or  furnishing  light  or  power.  Nevertheless, 
we  owe  to  electrochemical  operations  many  of  the  commonest  necessi- 
ties of  life.  The  commercial  applications  of  electrolysis  cover  a  wide 
and  useful  range,  from  nickel  and  silver  Plating  to  Electrotyping  for  the 
use  of  the  printer ;  and  from  methods  of  Bronzing  and  Gilding  to  meth- 
ods of  Smelting  certain  ores  and  Refining  metals.  Electrolysis  is  also 
becoming  a  most  important  factor  in  chemical  manufactories.  At  Niag- 
ara Falls  alone  not  less  than  10.000  horse  power  is  utilized  in  manu- 
facturing bleaching  powders  and  other  commercial  chemical  prod- 
ucts. Nearly  all  of  the  processes  depend  upon  the  laws  of  chemical 
action  which  have  already  been  described  in  Chapters  IV  and  V ;  but 
the  solutions  used  are  frequently  quite  complex,  so  that  the  chemical 
action  which  occurs  is  complicated  and  not  always  fully  understood. 

A  working  knowledge  of  the  processes  of  electro-deposition  of  metals 
has  been  possessed  only  since  1800,  and  indeed  many  of  the  more 
important  processes  of  electroplating,  electrotyping,  etc.,  have  been  dis- 
covered since  1840  or  1845,  while  some  of  the  important  operations  of 
electrometallurgy,  such  as  the  electrolytic  recovery  of  aluminum  and  the 
commercial  refining  of  copper  by  electrolysis,  have  not  been  employed 
until  within  a  very  few  years.  The  next  few  years  seem  destined  to  see 
electrolysis  and  electrometallurgical  processes  (processes  of  treating 

430 


ELECTROPLATING  43 1 

metals  in  which  electricity  is  used)  put  into  extended  use  in  the 
recovery  of  various  metals  from  their  ores,  and  in  some  hitherto  little 
explored  fields,  such  as  the  purifying  of  drinking  water  and  Sterilizing 
of  sewage. 

371 .  Electroplating ;  Silver  Plating.  —  Electroplating  is  the  process  of 
covering  articles  of  metal  with  a  thin  layer  of  another  metal  by  means  of 
electrolysis  from  a  solution  containing  a  salt  of  the  deposited  metal. 
The  covering  usually  consists  of  nickel,  silver,  or  gold,  and  the  base,  or 
covered  metal,  is  ordinarily  of  some  composition,  such  as  white  metal, 
Britannia  metal,  German  silver,  or  brass. 

The  details  of  the  processes  are  quite  different  for  the  different  met- 
als used  in  plating.  We  will  first  consider  silver  plating,  as  silver  is  the 
most  important  metal  in  plating  processes. 

The  commonest  salts  of  silver  are  chloride  of  silver,  nitrate  of  silver, 
cyanide  of  silver,  and  acetate  of  silver.  A  salt  of  a  metal  is  a  chemical 
combination  formed  by  the  action  of  an  acid  on  the  metal.  Thus,  ni- 
trate of  silver  is  formed  by  the  chemical  action  of  nitric  acid  upon  sil- 
ver. Nitric  acid  is  a  chemical  combination  of  hydrogen  with  oxygen 
and  nitrogen,  the  oxygen  and  nitrogen  in  this  case  forming  what  is  called 
an  add  radical.  The  radical  of  nitric  acid  has  a  greater  chemical  attrac- 
tion or  affinity  for  silver  than  for  hydrogen.  Consequently,  when  silver 
is  immersed  in  nitric  acid,  the  silver  is  attacked  and  dissolved,  during 
which  process  it  combines  with  the  acid  radical  and  forms  nitrate  of  sil- 
ver, while  the  hydrogen  of  the  acid  is  given  off.  The  salts  of  silver 
which  are  used  in  electroplating  are  usually  made  from  the  nitrate.  The 
nitrate  of  silver  is  produced  by  adding  pure  silver,  in  small  quantities 
at  a  time,  to  a  warm  mixture  of  one  measure  of  distilled  water  to  four 
measures  of  pure,  strong  nitric  acid.  The  action  of  the  acid  upon  the  sil- 
ver is  very  intense  and  causes  much  heat  to  be  given  off,1  and  if  the  mix- 
ture is  too  hot,  or  too  much  silver  is  added,  the  liquid  may  boil  over.  In 
this  case  the  mixture  may  be  cooled  by  adding  a  little  cold  distilled  water. 
When  the  mixture  will  dissolve  no  more  silver,  the  solution  may  be  put 
in  a  covered  jar  and  set  in  a  dark  place  until  it  is  required  for  use. 

A  properly  diluted  solution  of  nitrate  of  silver  is  used  with  a  silver 
voltameter,2  but  the  deposit  from  a  nitrate  solution  does  not  make  a  sat- 

1  Compare  the  action  of  sulphuric  acid  upon  copper,  Article  60.  2  Article  158. 


432 


ELECTRICITY  AND    MAGNETISM 


isfactory  plating.  The  best  silver-plating  solution  is  one  containing 
cyanide  of  silver.  Cyanide  of  silver  is  the  salt  formed  by  the  combina- 
tion of  silver  with  prussic  acid.  A  solution  of  cyanide  of  silver  is  formed 
by  slowly  adding  to  the  silver  nitrate  solution,  made  substantially  as 
already  described,  a  weak  solution  of  cyanide  of  potash,  or  white  prus- 
siate  of  potash,  as  it  is  often  called.  The  cyanide  of  potash  should  be 
dissolved  in  about  ten  times  its  own  weight  of  distilled  water.  The  addi- 
tion of  the  potash  solution  to  the  nitrate  of  silver  solution  should  be  con- 
tinued as  long  as  a  white  Precipitate  forms,  but  no  longer,  or  some  of  the 
silver  is  lost.  The  precipitate  which  forms  is  cyanide  of  silver.  This 
should  be  allowed  to  settle,  after  which  the  clear  liquid  may  be  carefully 
poured  or  drawn  off.  The  precipitate  is  then  washed  a  number  of  times 
by  pouring  distilled  water  over  it  and  stirring,  allowing  the  precipitate 
to  settle  and  pouring  off  the  liquid. 

Cyanide  of  silver  does  not  dissolve  in  water,  but  readily  dissolves  in  a 
solution  of  cyanide  of  potash  in  water,  and  silver-plating  solutions  are 
usually  made  by  so  dissolving  the  silver  cyanide.  Cyanide  solutions  are 
extremely  poisonous,  and  therefore  must  be  handled  carefully ;  and  on 
account  of  the  value  of  the  silver  which  they  contain,  must  be  handled 
without  waste. 

372.    Vats  for  Silver  Plating.  —  The  vats  in  which  silver  plating  opera- 
tions are  carried  out  are  usually  made  of  wood,  though  they  are  some- 
times made   of   sheet    iron  lined 
with  wood.     They  are  of  various 
dimensions,  but  generally  are  from 
two  to  three  feet  wide,  five  to  six 
feet  long,  and  about  thirty  inches 
deep.     When  the  solution  is  made 
up  and  put  in  the  vat  for  service, 
it  usually  does  not  require  chang- 
ing  for  a  number  of  years.      It 
sometimes  requires  filtering,  and 
the  addition  of  water  to  supply 
FIG.  325.  —  Silver  Plating  Vat  with  One  Side     that  lost  by  evaporation    or   the 
cut  away  so  as  to  show  the  Articles   in      addition   of  cyanide   galts   to    sup. 
the    Solution.      A,   A,  A,   Supports   for 

Anodes,  c,  c,  c,  c,  supports  for  Cathodes,     ply  losses  which  have  come  about 


ELECTROPLATING 


433 


by  electrolysis.  The  exact  proportions  of  the  solution  used  for  silver 
plating  in  different  factories  vary  considerably,  but  they  are  nearly 
always  substantially  as  already  described. 

The  general  arrangement  of  a  plating  vat  is  shown  in  Figure  325, 
where  the  flat  plates  inside  the  vat  are  sheets  of  silver  which  are  con- 
nected to  the  positive  pole  of  the  source  of  current  and  form  the  anodes 
of  the  electrolytic  cell,1  while  the 
spoons,   forks,  and  other  articles    ^ 
to  be  plated  form   the  cathodes. 
The  supports  for  the  anodes  and 
cathodes  are  usually  made  of  brass 


FIG.   326.  —  Method   of    supporting  Spoons 
and  Forks  in  Silver  Plating  Vat. 


or  copper  tubes  laid    across  the 
top  of  the  vat. 

The  articles  to  be  plated  are 
ordinarily  supported  on  looped 
pieces  of  insulated  copper  wire 
(Fig.  326).  The  insulation  of 

these  supports  where  they  are  immersed  in  the  liquid  is  important,  in 
order  to  avoid  an  unnecessary  and  expensive  deposit  of  silver  upon 
them.  The  silver  deposit  made  on  the  cathodes  occurs  as  a 'result 
of  electrolysis,  and  an  equal  amount  of  silver  goes  into  the  liquid  from 
the  anodes  when  all  is  working  well.2 

373.  Pressure  and  Current.  —  The  quality  of  the  deposit  which  is 
made  in  electroplating  is  of  the  first  importance.  The  three  points  to 
be  looked  after  most  carefully  are  :  the  strength  of  the  current  as  com- 
pared with  the  magnitude  of  the  surface  to  be  plated  ;  the  composition, 
density,  and  temperature  of  the  plating  solution ;  and  the  condition  of 
the  articles  to  be  plated  when  put  into  the  solution.  The  current  for 
plating  was  formerly  furnished  by  batteries,  but  it  is  now  ordinarily  fur- 
nished from  small  dynamos  which  produce  a  low  pressure  properly 
adapted  for  the  purpose.  The  pressure  may  also  be  adjusted  to  a  con- 
siderable extent  by  means  of  a  resistance  box  connected  in  a  circuit 
with  the  magnetizing  coils  of  the  dynamo.  The  current  and  pressure 
supplied  by  the  dynamo  may  be  measured  by  means  of  an  amperemeter 
and  a  voltmeter. 


Article  59. 


2  Article  60. 


2  F 


434  ELECTRICITY  AND   MAGNETISM 

One  dynamo  of  sufficient  size  may  be  used  to  supply  current  to  several 
plating  vats.  The  vats  may  be  connected  either  in  series  or  in  parallel, 
depending  upon  the  pressure  developed  by  the  dynamo.  When  the  cur- 
rent is  of  the  proper  amount,  the  silver  covering  which  is  deposited  upon 
the  plated  articles  is  hard,  white,  adheres  closely,  and  is  deposited  with 
reasonable  rapidity.  When  the  current  is  too  small,  the  deposit  is  usu- 
ally of  good  quality,  but  the  plating  progresses  too  slowly.  When  the 
current  is  too  great,  the  plating  is  likely  to  become  gray  or  black  and 
rough,  while  gas  is  sometimes  given  off  at  the  cathodes.  A  discolora- 
tion of  the  silver  deposit  may  also  occur  from  impurities  in  the  liquid. 
Such  discoloration  may  often  be  removed  by  proper  "  after  treatment " 
of  the  plated  articles,  but  to  this  attention  cannot  be  given  here. 

374.  Relative  Positions  of  Anodes  and  Cathodes. — The  form  of  the 
articles  to  be  plated  often  has  much  to  do  with  the  quality  of  the  plating. 
Thus,  bulky  articles  with  a  given  surface  often  cannot  be  plated  as 
rapidly  as  flatter  articles  with  exactly  the  same  amount  of  surface  to  be 
covered.  Edges  and  points  often  gather  a  granular  or  rough  deposit, 
while  the  flat  parts  of  the  same  articles  take  a  satisfactory,  hard  deposit. 
Such  difficulties  can  be  overcome  only  by  making  a  proper  mutual 
adjustment  of  the  distances  between  anodes  and  cathodes,  the  quality 
of  the  liquid,  and  the  current  per  unit  surface  of  the  articles.  When 
articles  which  have  great  irregularities  of  surface  are  to  be  plated,  the 
distance  between  anodes  and  cathodes  must  be  greater  than  that  which 
is  satisfactory  when  the  articles  have  a  uniform  surface,  otherwise  the 
more  prominent  points  of  the  articles  will  receive  a  heavy  deposit  while 
the  hollows  may  receive  little  or  no  deposit.  It  is  important  that  all 
plated  articles  be  given  a  uniform  deposit  of  proper  thickness  upon  the 
surfaces  which  are  desired  to  be  covered.  The  thickness  of  silver  plat- 
ing ordinarily  varies  from  the  thinnest  possible  coating  to  the  thickness 
of  thin  writing  paper,  depending  upon  the  quality  of  the  product. 

There  is  a  method  of  plating  by  simply  dipping  the  articles  in  a 
proper  silver  solution  which  is  used  to  silver  small  articles  such  as  hooks 
and  eyes,  on  which  the  coating  is  too  thin  to  be  really  measured.  In 
this  case  the  plating  is  not  due  to  electrolytic  action,  but  simply  to 
chemical  action  between  the  silver  solution  and  the  metal  composing 
the  articles  to  be  covered.  This  is  called  plating  by  simple  immersion. 


ELECTROPLATING 


435 


375.  Preparation  of  Material  for  Silver  Plating.  —  In  preparing  arti- 
cles for  silver  plating,  the  greatest  care  must  be  taken  to  make  them 
absolutely  clean  and  bright,  or  the  plating  will  not  take  a  permanent  hold, 
but  will  peel  off.     It  is  first  necessary  to  prepare  the  articles  for  the 
kind  of  coating  they  are  intended  to  receive ;  if  the  plating  is  intended 
to  be  polished,  the  articles  must  be  polished,  all  deep  scratches  must  be 
removed,  etc.      This  may  be  done  by  filing,  scouring,  polishing,  etc. 
After   this   preparation,  the  cleaning  is  begun  by  dipping  in   a  warm 
solution   of  caustic  potash  or  soda  which  cleans  off  all  grease.     This 
solution  is  made  by  dissolving  commercial  lye  in  water,  and  it  may  be 
used  continuously  until  its  caustic  properties  are  used  up. 

After  dipping  in  lye  the  articles  are  washed  in  water  and  are  then 
sometimes  dipped  in  dilute  acid  to  give  them  a  proper  surface.  They 
are  next  washed  with  great  care  and  then  placed  in  the  depositing  vat. 
It  is  quite  common  to  cover  articles  to  be  silver  plated  with  a  very  thin 
coating  of  mercury.  The  object  of  this  is  to  avoid  oxidation  of  the  arti- 
cles, which  causes  the  plating  to  peel.  Coating  with  mercury  is  called 
Quicking,,and  it  may  be  effected  by  dipping  the  articles  into  a  dilute  solu- 
tion of  nitrate  of  mercury,  or  the  solution  of  some  other  mercury  salt. 

During  the  operations  of  dipping,  the  articles  should  be  supported 
upon  wires  or  in  wire  baskets.  They  should  not  be  touched  with  the 
fingers,  since  the  points  so  touched  are  made  greasy,  and  the  deposit 
will  not  Take. 

376.  Buffing  and  Polishing.  —  After  the  plating  is  completed  in  the 
bath  the  articles  must  be  put  through  a  series  of  operations  to  give  the 
plated  surface  the  proper  finish.    This 

is  largely  done  by  polishing  on  rapidly 
revolving  wheels  made  of  brass  wires, 
leather,  and  canvas.  The  processes 
are  called  Scratching,  Buffing,  and 
Polishing.  The  same  tools  are  used 
for  polishing  the  articles  before  plat- 
ing. In  the  case  of  some  articles,  the 
polishing  is  done  by  means  of  hand 

Burnishers,  which  are  smooth  tools  made  of  steel,  agate,  or  similar  hard 
materials.     Some  forms  of  burnishers  are  shown  in  Figure  327. 


FIG.  327.  — Several  Forms  of  Steel 
Burnishers. 


436 


ELECTRICITY   AND   MAGNETISM 


the  Interior  of  a  Metal 
Cream  Pitcher. 


377.  Gold  Plating.  —  Plating  with  gold  is  carried  on  in  very  much 
the  same  way  as  plating  with  silver.     The  commonest  solution  is  of  cyanide 
of  gold  made  up  in  a  manner  quite  similar  to  that  used  in  making  up 
the  cyanide  of  silver  solution.     The  solution  is  generally  used  when  hot, 
and  great  care  to  have  all  the  details  exactly  right  is  necessary  to  get  a 
deposit  of  satisfactory  color.     It  is  particularly  important  that  all  the 

materials  used  in  making  the  solution  shall  be 
pure. 

Gilding  the  inside  of  silver  cups,  sugar  bowls, 
and  cream  pitchers  is  commonly  done  by  filling 

FIG.   328.  -  Arrangement      the   article   tO  be   Silded   with   the   hot   Solution, 

of  Circuit  for  Gilding  hanging  a  gold  anode  in  the  shape  of  a  cylinder 
in  the  centre  of  the  solution,  and  finally  con- 
necting up  a  battery  so  that  the  article  to  be 
gilded  is  the  cathode  (Fig.  328).  The  extreme  cleaning  of  articles 
for  gold  plating  is  usually  not  as  important  as  in  silver  plating,  since  the 
hot  solution  helps  in  the  cleaning. 

378.  Nickel  Plating.  —  Nickel  plating  is  probably  the  most   gener- 
ally used  of  all  the  different  styles  of  plating.     The  base  upon  which 
nickel  is  plated  is  usually  brass,  copper,  iron,  or  steel.     The  soft  white 
metals  which  are  often  silver  plated  are  seldom  nickel  plated.      The 
hardness  of  nickel  and  its  durable  polish  give  to  nickel  plating  great 
advantages  for  use  in  the  finish  of  sanitary  appliances,  car  fittings  and 
decorations,  small  nuts,  bolts,  screws,  chains,  etc.,  used  in  small  machin- 
ery, bicycles,  stove  fronts,   metal   lamps,   and  many 

similar  appliances. 

The  solution  which  is  used  in  nickel  plating  is  made 
from  a  combined  sulphate  of  nickel  and  sulphate  of 
ammonia.  In  a  dry  state,  this  is  ordinarily  known 
as  nickel  salts  or  the  double  sulphate  of  nickel  and 
ammonia.  This  double  salt  may  be  purchased  in  the 
market.  In  order  to  make  a  nickeling  solution,  the 
pure  salt,  which  comes  in  green  crystals,  is  obtained, 
and  is  dissolved  in  hot  water  at  the  rate  of  about  three-quarters  of  a 
pound  to  a  gallon  of  water.  The  vat  used  to  hold  a  nickeling  solution 
is  usually  of  wood  lined  with  lead.  The  joints  in  the  lead  lining  are 


FIG.  329.  —  Screws 
prepared  for  sus- 
pending in  Nickel 
Plating  Bath. 


ELECTROPLATING 


437 


FIG.  330.  —  Bicycle 
Spokes  prepared 
for  suspending 
in  Nickel  Plat- 
ing Bath. 


burned  together,  not  soldered.     In  the  preparation  of  articles  for  nickel 

plating,  t'hey  must  be  very  carefully  polished  and  cleaned  by  scouring, 

dipping  in  a  hot  lye  solution,  and  pickling  in  acid. 

The  latter  is  very  important,  since  the  acid  takes  off 

from  the  articles  the  thin  covering  of  oxide  which  is 

likely  to  stick  to  iron,  copper,  and  brass,  and  which 

the  nickel  solution  has  no  tendency  to  remove.     If 

the  oxide  covering  is  not  removed,  the  nickel  plate 

will  come  off,  or   Strip,  as   it    is   called,  while   the 

articles  are  being  burnished.     The  final  processes  of 

nickel  plating  are  polishing  and  burnishing. 

Articles  to  be  plated  with  nickel  are  hung  in  the 
•liquid  or  Bath  very  much  as   already  described  under  silver  plating. 
When  a  number  of  small  articles  are  to  be  nickel  plated  they  are  often 
^      n  suspended  in  a  string  from  the  same  wire.     Fig- 

ure 329  shows  the  manner  of  suspending  screws 
in  the  nickel  bath,  and  Figures  330  and  331 
show  the  manner  of  suspending  bicycle  spokes 
and  chains. 

Figure  332  shows  the  form  of  a  vat  generally 
used  in  nickel  plating.  Nickel  plating  vats  are 
generally  larger  than  those  used  in  silver  plating.  It  is  particularly  im- 
portant that  no  organic  (non-metallic)  impurities  be  allowed  in  a  nickel 
bath,  as  these  ruin  the  quality  of  a 
nickel  deposit. 

379.  Electrotyping  and  Copper  Plat- 
ing. —  Electrotyping  is  a  process  of 
reproducing  type  and  woodcuts  and 
other  illustrations  by  means  of  an 

electroplating    of    copper,    which    is    FlG.  332.  _  Nickel  Plating  Vat.    A  A, 
used  in  nearly  all  large   printing  es- 
tablishments.     In   electrotyping,    an 
Impression  or  mould  is  made  of  the 
type,  which  is  set  up  as  for  printing. 

This  mould  is  usually  made  in  wax  or  soft  paper  pulp  by  pressing  it 
hard   upon    the   type.      After   the   surface  of  the   mould   is  properly 


FlG.  331.— Small  Chain 
prepared  for  Nickel 
Plating  Bath. 


Rods  from  which  Nickel  Anodes 
may  be  suspended ;  B,  Rod  from 
which  Articles  to  be  plated  (Cath- 
odes) may  be  suspended. 


438  ELECTRICITY   AND    MAGNETISM 

trimmed  it  is  coated  with  fine  plumbago  or  some  similar  conductor, 
which  is  carefully  brushed  over  it,  so  that  an  electrolytic  shell  Of  copper 
may  be  deposited  upon  it. 

The  Plumbagoing,  as  it  is  called,  is  necessary  because  the  mould  itself 
is  a  non-conductor,  and  the  current  which  is  necessary  to  make  an  elec- 
trolytic coating  cannot  be  sent  through  it.  Plumbago  is  powdered 
graphite,  and  is  a  fairly  good  conductor,1  so  that  a  thin  coating  brushed 
over  the  surface  of  the  mould  enables  it  to  conduct  the  current.  Thus 
prepared,  the  mould  is  hung  in  an  electrolytic  bath,  consisting  of  a 
solution  of  sulphate  of  copper  (blue  vitriol)  to  which  a  small  percentage 
of  sulphuric  acid  is  added.  The  anode  of  the  electrolytic  cell  is  a  plate 
of  copper,  and  the  cathode  is  the  mould.  The  thickness  of  the  shell  of 
copper  which  is  deposited  on  the  mould  varies  from  that  of  a  sheet  of 
the  paper  upon  which  this  book  is  printed  to  several  times  that  thick- 
ness, depending  upon  how  much  printing  is  to  be  done  from  the  electro- 
typed  Plates. 

When  the  copper  deposit  is  of  proper  thickness,  the  mould  is  removed 
from  the  bath,  and  the  copper  shell  is  separated  from  the  mould.  The 
shell  is  then  trimmed,  and  finally  "backed  up"  by  a  filling  of  type 
metal,  which  is  melted  and  poured  upon  the  back  of  the  shell.  Electro- 
typed  plates  have  a  great  advantage  over  type,  in  having  a  permanent 
form  and  in  wearing  much  better.  As  soon  as  the  mould  for  electro- 
typing  is  taken  off  from  set-up  type,  the  type  may  be  distributed  and 
used  again. 

Copper  plating  is  also  sometimes  used  to  give  a  bronze  finish  to  iron 
lamp-posts,  gas  fixtures,  etc.,  and  it  is  used  to  make  a  foundation  coat- 
ing upon  iron  articles  which  it  is  desired  to  silver  plate.  The  solution 
which  is  used  for  this  purpose  is  usually  the  same  as  that  used  for 
electrotyping. 

380.  Plating  with  Other  Metals.  —  Plating  with  other  metals  man 
those  referred  to  above,  such  as  iron,  tin,  or  zinc,  is  sometimes  carried 
out  for  special  purposes.  These  require  special  solutions  and  peculiar 
care  in  handling  the  articles  to  be  plated.  It  is  even  possible  to 
electrolytically  deposit  brass,  german  silver,  or  other  alloys.  These 
require  extreme  care,  however,  in  the  management  of  the  solutions 

i  Article  7. 


ELECTRIC   REFINING   OF   METALS  439 

and  the  regulation  of  the  electric  pressure  and  current  supplied  to 
the  vats. 

Zinc  plating  has  come  into  considerable  vogue  of  late  years  as  a  sub- 
stitute for  Galvanizing,  in  which  articles  are  covered  with  a  zinc  coating 
by  dipping  them  in  a  bath  of  melted  zinc. 

381.  Refining  Copper.  —  A  very  useful  application  of  electrometallurgy 
is  the  refining  of  copper.  In  this  operation  the  Crude  Copper  which 
comes  from  ordinary  smelting  works  with  from  two  to  five  per  cent  of 
impurities  is  refined  by  electrolysis  so  that  only  very  minute  amounts  of 
impurities  remain.  We  have  already  seen l  the  effect  of  impurities  in 
reducing  the  electrical  conductivity  of  copper  and  other  metals.  When 
the  electrolytic  method  of  refining  copper  is  properly  carried  out,  it 
leaves  such  a  small  amount  of  impurities  that  the  Electrolytic  Copper  has. 
almost  as  great  conductivity  as  pure  copper. 

Copper  wires  to  be  used  in  electric  lighting  and  in  the  manufacture 
of  electric  machines  are  therefore  generally  made  of  electrolytic  copper. 
It  is  usual  for  such  wires  to  have  more  than  98  per  cent  of  the  conduc- 
tivity of  pure  copper.  The  small  amount  of  impurities  which  do  remain 
in  electrolytically  refined  copper  is  largely  composed  of  silver  and  iron. 

In  electrolytic  refining,  the  crude  copper  is  cast  into  heavy  plates 
which  are  used  as  anodes  in  depositing  vats.  The  solution  in  these 
vats  is  copper  sulphate  with  a  little  sulphuric  acid.  The  cathodes  at 
first  are  thin  sheets  of  pure  copper,  but  they  grow  by  deposition  into 
thick  plates  of  copper  which  may  be  worked  into  bars  and  drawn  into 
wires  as  desired.  The  action  in  the  depositing  vats  is  quite  similar  to 
that  which  goes  on  in  a  copper  voltameter. 

Enormous  dynamos  are  used  in  copper  refining  works,  and  a  great 
number  of  tanks  or  vats,  each  containing  a  number  of  anodes  and 
cathodes  which  are  arranged  in  alternate  rows,  are  provided.  The  vats 
are  ordinarily  connected  in  series,  or  sets  of  a  number  of  vats  connected 
in  series  are  connected  in  parallel  with  each  other.  The  several  rows 
of  anodes  in  each  vat  are  connected  in  parallel,  as  are  also  the  cathodes. 
The  pressure  required  to  pass  the  current  through  each  vat  is  quite 
small,  and  consequently  a  number  of  vats  may  be  connected  in  series 
without  causing  the  total  pressure  to  exceed  100  volts. 

1  Article  91. 


440  ELECTRICITY   AND    MAGNETISM 

It  is  desirable  that  the  pressure  required  at  each  vat  be  as  little  as 
possible,  in  order  to  avoid  the  deposition  of  impurities  on  the  cathodes, 
and  also  to  save  power.  The  power  used  in  each  vat  is  equal  to  the 
difference  of  pressure  between  the  anodes  and  cathodes  multiplied  by 
the  current  flowing  through  the  vat. 

It  is  desirable  to  have  as  great  a  current  flow  as  will  give  a  fairly 
smooth  deposit,  in  order  that  the  time  required  in  depositing  each 
pound  of  copper  may  be  as  small  as  possible.  Any  reduction  made  in 
the  current  without  changing  the  pressure  simply  reduces  in  a  propor- 
tional rate  the  amount  of  copper  deposited  per  hour,  so  that  the  power 
required  to  deposit  a  pound  of  copper  is  not  materially  changed.  If  the 
pressure  required  to  pass  the  current  through  the  vats  is  reduced  with- 
out changing  the  current,  it  at  once  reduces  the  power  required  to 
deposit  a  pound  of  copper,  and  a  saving  in  the  cost  of  manufacture  is 
effected.  In  order  to  reduce  the  pressure  the  anodes  and  cathodes  are 
set  as  closely  together  as  possible  without  interfering  with  the  circula- 
tion of  the  solution. 

During  the  process  of  refining  copper  by  this  means  the  impurities  of 
the  crude  copper  are  mostly  dissolved  in  the  solution  or  are  thrown  to 
the  bottom  of  the  vats  as  mud  or  Sludge.  Copper  that  is  to  be  electro- 
lytically  refined  usually  contains  some  silver  and  a  little  gold.  During 
the  refining  process  these  form  salts,  which  become  part  of  the  mud,  and 
the  precious  metals  are  recovered  by  the  ordinary  method  of  smelting 
when  the  mud  is  removed  from  the  vats.  Iron  and  lead  are  also  con- 
tained in  the  crude  copper,  as  are  smaller  quantities  of  other  metals. 
The  lead  goes  to  the  bottom  of  the  tank  like  the  gold  and  silver,  and 
the  iron  dissolves  in  the  solution,  but  is  not  deposited  on  the  cathode 
except  in  very  small  amounts  unless  the  pressure  at  the  vat  is  too  high. 
Electrolytic  refining  of  crude  copper  from  ores  which  contain  silver  is 
particularly  useful,  because  it  is  the  cheapest  method  of  separating  the 
silver  from  the  copper.  Great  electrolytic  refineries,  therefore,  have 
been  erected  at  the  Montana  copper  mines,  the  ores  of  which  usually 
contain  a  valuable  portion  of  silver. 

382.  Refining  Other  Metals.  —  Electrolysis  has  been  applied  to  the 
refining  of  other  metals,  but  without  great  success.  It  has  also  been 
used  in  the  recovery  of  precious  metals  from  their  ores,  but  thus  far 


ELECTRICAL   PRODUCTION   OF   ALUMINUM  44! 

without  much  success,  although  there  is  every  reason  to  believe  that  it 
may  ultimately  prove  of  value  in  working  certain  kinds  of  ores. 

383.  Electrolytic  Reduction  of  Aluminum.  —  A  most  important  joint 
application  of  electric  heating  and  electrolysis  is  now  used  in  the  pro- 
duction of  the  valuable  metal  aluminum.  The  compounds"  of  this  metal 
compose  an  amazing  proportion  of  the  material  of  the  earth's  crust; 
they  exist  in  the  forms  of  clay,  marl,  slate,  feldspar,  mica,  corundum, 
and  many  other  mineral  forms  which  go  to  make  a  part  of  the 
rock  material  of  the  earth.  These  compounds  contain  goodly  propor- 
tions of  the  metal  itself,  though  it  is  never  found  in  a  native  metallic 
state ;  and  if  all  the  aluminum  in  the  compounds  were  recovered,  it 
would  be  found,  so  it  is  estimated,  to  compose  as  much  as  one-twelfth 
part  of  the  bulk  of  the  earth. 

With  all  this  wide  prevalence  of  the  compounds,  and  a  recognition  of 
the  value  of  the  metal,  no  commercially  satisfactory  means  of  recover- 
ing it  from  the  compounds  were  devised  until  very  recent  years,  when 
certain  electrical  processes  came  into  use.  The  process  which  is  now 
universally  used  was  almost  simultaneously  discovered  in  1886  by  Hall 
in  the  United  States  and  Heroult  in  France. 

Aluminum  cannot  be  electrolytically  deposited  from  solutions  of  its 
salts  in  water,  as  the  metal  at  once  becomes  oxidized  at  the  cathode, 
and  some  other  solvent  must  be  used.  In  the  process  of  Hall  and 
Heroult  this  solvent  is  a  melted  "  bath  "  of  the  mineral  cryolite,  and  the 
aluminum  compound  which  composes  the  electrolyte  in  this  bath  is  an 
oxide  of  aluminum,  or  alumina.  Fortunately  for  the  process,  beds  of  a 
reasonably  pure,  natural  aluminum  oxide  called  bauxite  are  found  in  sev- 
eral parts  of  the  world.  Alumina  is  also  a  "  by-product  "  of  some  chemi- 
cal manufacturing  processes. 

In  the  operation  of  the  process,  melted  cryolite  is  put  into  large  iron 
pots  which  are  lined  with  a  hard,  baked  carbon  mixture,  and  carbon  an- 
odes are  dipped  into  the  bath.  The  carbon  lining  of  each  pot  serves  as 
cathode.  An  electric  current  of  about  2500  amperes  is  passed  through 
each  bath,  with  a  pressure  between  the  terminals  of  each  pot  of  eight 
or  nine  volts.  This  represents  sufficient  power  to  heat  the  bath  and 
keep  the  cryolite  in  a  molten  condition. 

The  aluminum  oxide  is  dissolved  in  this  bath  of  molten  cryolite,  and 


442 


ELECTRICITY   AND    MAGNETISM 


the  electric  current  causes  it  to  separate  into  its  constituents  —  aluminum 
and  oxygen.  The  aluminum  goes  with  the  current  to  the  cathode,  and 
lies  in  a  melted  condition  in  the  bottom  of  the  pot  until  siphoned  off. 
The  oxygen  goes  to  the  positive  pole  or  anode,  and  combines  with  the 
carbon,  which  is  gradually  consumed.  After  the  melted  aluminum  has 

been  drawn  off  from  the  pot,  it  is  cast 
into  ingots,  from  which  it  may  be  made 
into  sheets,  rods,  wires,  or  other  forms, 
as  may  be  desired. 

Many  thousands  of  horse  power  are 
used  in  the  manufacture  of  aluminum  at 
Niagara  Falls,  and  equally  great  powers 
are  utilized  elsewhere  in  the  world  ;  and 
the  demand  for  the  metal  is  growing 
with  a  truly  wonderful  rapidity  as  the 
price  becomes  more  reasonable.  The 
cheapest  aluminum  ore,  clay,  has  not 
been  made  available  for  use  by  any 
process  of  recovery  yet  discovered. 

The  direct  current  must  be  used  in 
this  process,  as  in  other  electrolytic 
processes ;  and  where  alternating  cur- 
rent is  supplied,  as  at  Niagara  by  the 

Niagara  Falls  Power  Company,  it  must  be  transformed  into  direct  cur- 
rent by  means  of  rotary  transformers  before  it  can  be  used  in  the  elec- 
trolytic process.  One  of  the  electrolysis  vats  used  by  the  Pittsburg 
Reduction  Company,  for  the  production  of  aluminum  at  Niagara  Falls  is 
illustrated  in  Figure  333. 

384.  Smelting  by  the  Electric  Arc.  —  The  electric  arc,  such  as  is  seen 
in  arc  lamps,  but  very  much  larger,  has  been  successfully  used  in  work- 
ing ores  and  producing  chemical  changes.  This  application  is  usually 
called  Electric  Smelting.  The  action  which  occurs  in  electric  smelting 
is  mostly  due  to  chemical  action  set  up  by  the  intense  heat  of  the  elec- 
tric arc  which  melts  the  ores.  It  was  by  electric  smelting  from  corundum 
and  similar  material  that  the  Cowles  Company, of  Lockport,  N.Y.,  made  the 
aluminum  which  was  found  in  their  formerly  well-known  aluminum  bronze- 


FIG.  333.  —  Vat  used  in  the  Recovery 
of  Aluminum  from  Alumina.  C, 
C,  C,  Carbon  Anodes  suspended 
in  Bath  of  Molten  Cryolite;  A, 
Iron  Crucible  which  contains 
Bath,  and  serves  as  Cathode. 


ELECTRIC   SMELTING  443 

In  the  process  of  electric  smelting,  the  ore,  generally  mixed  with  car- 
bon, is  placed  between  great  carbon  electrodes  in  a  carbon-lined  furnace 
built  out  of  fire-brick,  or  composed  of  an  iron  box. 

A  current  of  hundreds,  or  perhaps  thousands,  of  amperes  is  sent 
between  the  electrodes,  and  the  heating  is  so  violent  that  the  ore  is 
fused,  or  even  volatilized,  with  an  accompanying  vigorous  chemical 
action. 

The  first  use  of  the  high  temperature  obtainable  by  the  electric  arc 
for  chemical  purposes  seems  to  have  been  made  in  some  experiments  by 
Depretz  in  1849.  Although  several  furnaces  were  made  soon  after,  the 
real  impulse  to  this  line  of  investigation  was  given  in  1880  by  a  paper  of 
Sir  William  Siemens  describing  his  own  work. 

The  importance  of  the  resulting  developments  cannot  be  well  overesti- 
mated. They  have  already  given  us  calcium  carbide,  from  which  acety- 
lene gas  is  produced,  carborundum,  which  is  the  hardest  manufactured 
substance,  and  many  other  compounds  which  hitherto  could  only  be 
manufactured  at  a  prohibitory  cost. 

385.  Calcium  Carbide.  —  Calcium  carbide,  which  is  used  for  charging 
the  acetylene  generators  used  for  lighting  purposes,  is  made  by  electric 
smelting.     In  1892,  Moissan,  in  France,  wrote  :  "  If  the  temperature  of 
the  electric  furnace  reaches  3000°,  the  material  of  the  furnace  itself,  the 
quick-lime,  melts  and  runs  like  water.     At  this  temperature  the  carbon 
rapidly  reduces  the  oxide  of  calcium  (lime),  and  the  metal  (calcium) 
is  set  free  in  abundance.     It  combines  easily  with  the  carbon  of  the 
electrodes  to  form  carbide  of  calcium,  fluid  at  this  heat,  which  is  easily 
recovered."     This  statement  gave  to  the  world  the  discovery  of  a  valu- 
able process  for  which   Moissan  probably  deserves  full  credit,  though 
Thomas  L.  Wilson,  of  the  United  States,  is  supposed  to  have  made  the 
discovery  at  about  the  same  time. 

To  make  the  carbide,  powdered  lime  and  coke  are  thoroughly  mixed 
and  placed  in  an  electric  furnace  similar  in  principle  to  that  described 
in  the  previous  article.  When  the  mass  is  brought  to  a  sufficiently  high 
heat,  the  calcium  of  the  lime  combines  with  the  carbon,  forming  calcium 
carbide.  This  carbide,  when  put  into  water,  decomposes,  and  acetylene 
gas  is  formed. 

386.  Carborundum.  —  This  product  of  the  electric  furnace  takes  a 


444  ELECTRICITY   AND    MAGNETISM 

place  with  emery  and  corundum  as  a  grinding  and  abrading  material  on 
account  of  its  extraordinary  hardness.  It  is  a  chemical  combination  of 
carbon  and  silicon,  called  silicon  carbide,  which  is  made  in  the  electric 
furnace  by  fusing  a  mixture  of  clean  sand  and  powdered  carbon.  Upon 
cooling  the  furnace,  the  carborundum  is  found  in  beautiful  masses  of 
lustrous,  dark  crystals.  The  character  of  the  furnace  used  in  its  manu- 
facture does  not  differ  greatly  from  that  described  in  the  preceding 
article. 

387.  Miscellaneous  Applications.  —  The  applications  of  electrolytic 
processes  and  electric  smelting  are  becoming  very  widespread,  and  are 
rapidly  taking  on  an  aspect  of  the  utmost  value.     Electrolysis  is  being 
used  in  the  large  cotton  mills  and  paper  mills  for  the  production  of 
bleaching  solutions ;   it  is  used  in  certain  establishments   for  cleaning 
metal  products,  and,  on  the  whole,  its  value  as  a  process  in  manufactur- 
ing has  apparently  only  begun  to  be  appreciated,  and  the  most  rapid 
extension  of  its  use  may  be  confidently  predicted.     The  rapid  extension 
of  electric  smelting  to  the  treatment  of  numerous  metal  ores  and  the 
manufacture  of  additional  chemical  products  may  be  anticipated  with 
equal  confidence. 

388.  Electric  Forging.  —  In  some  of  the  preceding  articles  it   has 
been  shown  that  the  heat  that  may  be  developed  from  electrical  sources 
is  of  great  service  in  metallurgy.     It  is  also  applied  in  many  simple 
processes,  such  as  welding  metals,  cooking,  etc. 

The  use  of  the  electric  current  for  heating  and  working  metals  is  not 
new.  As  early  as  1865  patents  were  issued  relating  to  the  subject; 
but  on  account  of  the  great  expense  of  the  current  generated  by 
batteries,  these  early  endeavors  came  to  naught,  and  not  until  within 
a  very  few  years  has  electric  metal  working  been  made  an  actual 
success.  It  was  as  late  as  1888  before  electric  welding  was  applied 
to  commercial  uses,  but  immediately  upon  its  introduction  it  came 
rapidly  into  favor,  and  even  created  much  excitement  among  some 
manufacturers. 

Electrical  methods  are  now  used  for  welding,  brazing,  heating,  shap- 
ing, and  tempering  metals.  For  most  of  these  purposes  the  method  in 
common  use  is  to  pass  an  electrical  current  of  very  great  volume  through 
the  metal  to  be  worked.  This  great  current  generates  sufficient  heat  as 


ELECTRIC  FORGING 


445 


it  passes  through  the  resistance  of  the  metal  to  quickly  raise  the  tem- 
perature to  a  welding  or  bending  heat,  or  even  to  melt  the  metal.  This 
method  of  heating  has  an  advantage  over  the  ordinary  method  of  heat- 
ing in  the  forge  fire,  which  heats  a  piece  of  metal  from  the  outside, 
while  the  electrical  method  heats  all  parts  of  the  metal  equally  and  at 
the  same  time  the  metal  remains  perfectly  clean. 

The  apparatus  which  is  used  for  heating  usually  consists  of  an 
alternating  current  transformer,  which  reduces  the  line  pressure  of  an 
alternating  current  to 
one  or  two  volt^,  or 
even  less,  and  in- 
creases its  volume  pro- 
portionally.1 A  welder 
transformer  is  shown 
in  Figure  334.  The 
grooved  copper  cast- 
ing shown  in  the  figure 
is  the  secondary  coil  of 
the  transformer,  which 
has  only  one  turn.  The 
primary  winding,  made 
up  of  numerous  turns 
of  wire,  is  intended  to 
lie  in  the  groove  of 
the  secondary,  while 

the  core,  which  is  seen  enclosing  one  side  of  the  secondary  casting, 
embraces  both  coils.  At  the  top  of  the  secondary  casting  are  sliding 
clamps  in  which  the  metal  to  be  heated  is  fastened. 

Electric  welding,  as  ordinarily  carried  on,  consists  of  heating,  by  the 
process  above  described,  the  pieces  of  metal  to  be  welded  while  they 
are  firmly  butted  against  each  other.  When  the  metals  have  been 
heated  till  they  -are  soft  at  the  points  in  contact,  they  are  squeezed 
together  a  certain  amount,  the  current  is  shut  off,  and  the  .weld  is  com- 
plete. This  is  the  process  developed  so  usefully  by  Professor  Elihu 


FIG.  334.  —  Alternating  Current  Transformer  of  Thomson 
Electric  Welder.  S,  S,  S,  Copper  Secondary  Coil ;  C, 
Laminated  Core  of  Transformer. 


1  Article  239. 


446 


ELECTRICITY   AND    MAGNETISM 


Thomson.  The  apparatus  which  is  generally  used  in  the  Thomson 
welding  process  is  :  i,  an  alternator,  usually  giving  a  frequency  of  from 
40  to  60  periods  per  second ;  2,  a  welding  transformer  with  clamps 
(Fig.  334)  and  arrangements  for  automatically  making  the  welds; 
3,  apparatus  for  controlling  the  amount  of  current  supplied  to  the 
transformer. 

Figure  335  shows  a  complete  Thomson  welder.     The  transformer  is 
seen  in  the  centre  of  the  case,  and  the  clamps  on  top.     The  weights  at 


Vic,.  335.  —  Thomson  Automatic  Electric  Welder. 


the  left  are  for  squeezing  together  the  heated  rods  held  in  the  clamps, 
and  the  relay  shown  at  the  right-hand  side  of  the  transformer  is  for 
cutting  off  the  current  when  the  weld  is  completed.  In  welding  heavy 
work,  hydraulic  pressure  is  used  to  squeeze  the  weld. 

Many  metals  may  be  welded  by  the  electrical  method  which  cannot 
be  coaxed  into  a  weld  by  the  ordinary  methods  of  the  blacksmith. 
Metals  which  have  been  welded  by  the  Thomson  process  are  named  in 
the  accompanying  table  :  — 


ELECTRIC   FORGING 


447 


Wrought  Iron. 
Cast  Iron. 
Malleable  Iron. 
Various    grades 
of  Tool  Steel. 
Various    grades 
of  Mild  Steel. 
Steel  Castings. 
Chrome  Steel. 
Wrought  Copper. 

Cast  Copper. 
Lead. 
Musshet's  Steel. 
Stubb's  Steel. 
Crescent  Steel. 
Bessemer  Steel. 
Tin. 
Zinc. 
Antimony. 
Wrought  Brass. 

Cast  Brass. 
Gun  Metal. 
Brass  Composi- 
tion. 
Cobalt. 
Nickel. 
Bismuth. 
Fuse  Metal. 
Type  Metal. 
Solder  Metal.    • 

German  Silver. 
Silver. 
Platinum. 
Aluminum  al- 
loyed   with 
Iron. 
Aluminum 
Brass. 
Aluminum 
Bronze. 

Aluminum. 
Phosphor 
Bronze. 
Gold  (pure). 
Manganese. 
Magnesium. 
Silicon  Bronze. 
Coin  Silver. 
Various    grades 
of  Gold. 

Again,  many  of  these  metals  may  be  welded  to  each  other  in  com- 
bination. The  combinations  which  have  been  made  are  shown  in  the 
table  below.  In  each  of  the  cases  where  a  weld  can  be  made  at  all,  it 
becomes  practically  as  strong  as  the  metal  itself. 


COMBINATIONS 


Copper  to  Brass. 
Copper  to  Wrought 

Iron. 
Copper  to  German 

Silver. 

Copper  to  Gold. 
Copper  to  Silver. 
Brass  to  Wrought 

Iron. 

Brass  to  Cast  Iron. 
Tin  to  Zinc. 


Tin  to  Brass. 
Brass     to    German 

Silver. 

Brass  to  Tin. 
Brass  to  Mild  Steel. 
Wrought     Iron    to 

Cast  Iron. 
Wrought     Iron    to 

Cast  Steel. 
Wrought     Iron    to 

Mild  Steel. 


Wrought  Iron  to  Tool 
Steel. 

Gold  to  German  Sil- 
ver. 

Gold  to  Silver. 

Gold  to  Platinum. 

Silver  to  Platinum. 

Wrought  Iron  to 
Musshet's  Steel. 

Wrought  Iron  to 
Stubb's  Steel. 


Wrought    Iron    to 

Crescent  Steel. 
Wrought     Iron    to 

Cast  Brass. 
Wrought     Iron    to 

German  Silver. 
Wrought     Iron    to 

Nickel. 
Tin  to  .Lead. 


A  very  striking  application  of  electric  welding  has  been  adopted  by  at 
least  one  manufacturer  for  welding  together  the  parts  of  street  railway 
track  material,  such  as  switches,  frogs,  etc.,  which  are  ordinarily  made 
up  by  bolting  together  pieces  of  rails  cut  to  proper  shape.  By  the  weld- 
ing process  bolts  may  be  dispensed  with,  and  the  work,  therefore,  is 
made  much  more  substantial.  A  process  has  even  been  developed  for 
welding  the  rails  of  a  street  railway  track  together,  thus  doing  away  with 
the  usual  bolted  joints  which  cause  so  much  roughness  in  the  track  and 
require  such  a  large  expense  for  repairs.  Figure  336  shows  a  track- 
welding  outfit.  This  welder  is  arranged  to  work  on  track  which  is  in 


448 


ELECTRICITY   AND    MAGNETISM 


place  in  the  street.     The  current  is  supplied  to  it  by  a  rotary  transformer 
which  transforms  the  5oo-volt  continuous  current  taken  from  the  trolley 


FIG.  336.  —  Electric  Welder  welding  the  Joints  in  Street  Railway  Rails. 

wire  into  an  alternating  current  at  a  pressure  of  about  350  volts.  Figure 
337  shows  a  complete  weld  at  a  rail  joint.  As  much  as  250  horse  power 
is  required  for  a  few  seconds  in  making  such  a  large  weld. 

389.  Welding  a  Ring.  —  One  of  the  striking  things  about  Thomson 
electric  welders,  is  their  ability  to  weld  up  rings ;  and  the  welders  may 
therefore  be  used  in  welding  wagon  tires,  chain  links,  etc.  In  this  case, 
the  question  occurs,  "Why  does  the  electric  current  not  flow  around 
through  the  solid  metal  from  clamp  to  clamp,  instead  of  through  the 
path  where  the  ends  of  the  ring  butt  against  each  other?"  This  is 
simply  a  question  of  electrical  impedance.1  In  the  case  of  a  wagon  tire, 
the  alternating  current  furnished  by  the  welder  transformer  would  have 
to  flow  through  a  path  several  feet  long  in  going  from  clamp  to  clamp 
through  the  solid  metal,  while  the  path  through  the  point  to  be  welded 

1  Article  236. 


ELECTRIC   FORGING 


449 


is  only  a  few  inches  in  length,  so  that  the  latter  path  is  of  much  the 

least  impedance,  and  nearly  all  of  the  current  follows  it.     The  long 

part  of  the  tire,  being  almost  a  complete  turn,  will  have  a  large  back 

pressure   set   up   in   it   due   to   its   self-inductance.     In  a  very  small 

ring,    enough    current 

may  pass  between  the 

clamps     through     the 

solid  part  of  the  ring 

to  heat  it  red  hot,  but 

that  does  not  interfere 

with  the  welding. 

390.  Softening   Ar- 
mor  Plate.  —  An    in- 
teresting     application 
of  the  Thomson  proc- 
ess   has    been    lately 
made  to  softening,  at 

points  where  bolt  holes          RG  337._  We]ded  Joint  in  Street  Railway  Track> 
must    be    drilled,    the 

very  hard  nickel-steel  armor  plates  which  are  made  for  United  States 
men-of-war.  The  plates  are  so  hard  that  it  is  almost  impossible  to 
drill  them  as  they  come  from  the  steel  works,  but  by  means  of  an 
electric  heating  arrangement  they  are  softened  at  the  spots  where  the 
bolt  holes  must  be  made,  without  injuring  the  temper  of  the  other  parts 
of  the  plates. 

391.  Arc  Welding.  —  Another  process  of  utilizing  the  heating  effect 
of   electricity  for  the  purpose  of  welding  and  working  metals,  is  that 
known  as  the  Arc  Process.     This  was  first  used  by  De   Meritens,  a 
Frenchman,  and  was  later  more  fully  developed  by  a  Russian  named 
Bernardos.     In  this  process,  a  continuous  current  is  used  at  a  pressure 
of  about  150  volts,  one  terminal  of  the  electric  generator  being  connected 
to  the  metal  which  it  is  desired  to  heat,  and  the  other  terminal  being 
attached  by  a  flexible  conductor  to  a  portable  carbon  rod  (Fig.  338). 
When  the  carbon  rod  is  brought  against  the  work,  an  electric  arc  is  set 
up  and  the  metal  is  heated. 

This  device  has  been  used  in  the  process  of  filling  with  metal  the  blow 

2G 


450  ELECTRICITY    AND   MAGNETISM 

holes  which  sometimes  occur  in  valuable  castings.  It  has  also  been 
used  for  welding  the  seams  in  small  iron  boilers,  receivers  for  compressed 
air,  and  other  iron  vessels.  It  is  of  special  advantage  for  the  latter  work, 


SHEET  IRON  SHIELD  TO  PROTECT  WORKMAN'S  HANDS 
,/"  \, 


FiG.  338.  —  Carbon   Rod  mounted  on  Handle,  with  Protecting  Shield  for  Use  in  "Arc 
Process  "  of  working  Metals. 

since  the  arc  can  be  slowly  drawn  along  the  edges  of  the  plates  to  be 
welded,  thus  bringing  them  to  a  welding  heat,  and  the  weld  is  then  com- 
pleted by  pressing  or  hammering  the  edges  together. 

392.  Pail  Welding.  —  In  each  of  the  methods  of  electric  welding  it 
is  to  be  noticed  that  the  electric  current  is  used  only  for  the  purpose  of 
heating  the  product  previous  to  welding,  and  that  the  mechanical  press- 
ure required  to  complete  the  weld  is  applied  by  external  means  of  some 
kind.  There  is  another  striking  and  even  startling  metho.l  of  electrically 
heating  metals  for  purposes  of  working  them.  If  a  pail  of  water,  in 
which  is  dissolved  some  common  washing  soda,  has  immersed  in  it  a 
lead  plate  which  is  connected  to  the  positive  terminal  of  a  150  or  200 
volt  electric  circuit,  it  gives  all  the  apparatus  necessary  for  quickly 
heating  iron.  The  metal  to  be  heated  is  grasped  in  tongs  which  are 
electrically  connected  to  the  negative  terminal  of  the  electric  circuit, 
the  handles  of  the  tongs  being  insulated. 

When  the  metal  is  plunged  into  the  pail  of  water,  it  is  quickly  brought 
to  a  white  heat  and  may  then  be  withdrawn  and  worked  on  the  anvil  or 
welded  to  another  piece  of  heated  iron  by  the  ordinary  blacksmith's  method. 
Any  metal  may  be  heated  by  this  process,  but  welding  can  be  performed 
only  on  those  metals  which,  like  iron,  can  be  welded  by  the  blacksmith. 
The  heating  of  the  metal  when  it  is  plunged  into  the  water,  is  apparently 
caused  by  an  electric  arc  which  is  set  up  around  the  submerged  metal 
on  account  of  its  becoming  surrounded  by  a  coating  of  hydrogen  gas. 
The  amount  of  current  used  varies  from  a  few  amperes  to  many  him- 


ELECTRIC   HEATING   AND   COOKING 


451 


dreds,  depending  upon  the  size  of  the  metal  to  be  heated.     It  is  a 
remarkable  sensation  to  see  a  piece 
of  metal  which  is  dipped  into  a  pail 
of  water  come  quickly  to  a  blinding 
white  heat ;  and,  when  held  in  an- 
other pair  of  tongs  (not  connected 
to  the  electric  circuit),  to  see  the  same    Fl"G>  339._Common  Form  of  E]ectTic 
piece  of  metal  again  dipped  in  the  same  Heater. 

wafer  for  the  purpose  of  cooling  it. 

393.    Cooking  by  Electricity.  —  The  direct  heating  effect  of  an  electric 

current  as  it  passes  through 
resistance  coils  is  now  applied 
to  the  ordinary  purposes  of 
warming  and  also  to  cook- 
ing. Figure  339  shows  one 
of  the  various  forms  of  elec- 
tric heaters,  all  of  which  sim- 
ply consist  of  resistance  wires 
embedded  in  an  insulating 
material.  These  heaters  are 
used  quite  generally  for  warm- 
ing electric  cars,  and  are  com- 
ing into  more  or  less  use  in  other  situations.  Electric  tea-kettles,  which 
are  kettles  with  an  electric 
heater  in  their  base,  are 
not  uncommon.  Electric 
flat-irons,  curling  irons,  sol- 
dering irons,  and  similar  de- 
vices are  slowly  making  their 
way  into  some  use  in  towns 
where  incandescent  electric 
light  circuits  are  at  hand 
to  supply  the  necessary  cur- 
rent. In  Figures  340,  341,  FlG-  34L  —  Electric  Curling-iron  Heater  with  Curl- 
ing Iron. 

and  342  are  shown  an  elec- 
tric sauce-pan,  an  electric  curling-iron  heater,  and  a  smoothing  iron. 


FlG.  340.  —  Electric  Sauce-pan.  A  is  a  flexible 
conducting  cord  which  rnay  be  extended  for 
connecting  the  sauce-pan  with  an  electric 
circuit. 


452 


ELECTRICITY   AND    MAGNETISM 


Whole  electric  kitchen  outfits  may  be  obtained,  including  an  electric 
range,  and  they  are  sure  to  come  into  quite  common  use  if  their  cost 

becomes  reduced  to  about  that  of 
coal  ranges.  The  arrangement  of 
a  complete  kitchen  outfit  is  shown 
in  Figure  343. 

While  electric  cooking  may  be 
said  to  be  satisfactory  on  account 
of  its  convenience,  cleanliness,  and 
adaptability,  electric  heating  for  gen- 
eral purposes  can  never  replace  the 
direct  use  of  coal  or  the  use  of 
steam  heating,  until  electricity  is 
directly  generated  from  the  fuel  or 
its  equivalent  without  the  interven- 
tion of  steam  engines  in  which  en- 
ormous losses  of  heat  cannot  be 
prevented.  The  nature  of  steam 
engines  makes  it  impossible,  even 
with  the  best  of  them,  to  convert 
into  useful  power  more  than  10  or 
15  per  cent  of  the  heat  energy  con- 
tained in  the  coal  which  is  shovelled 

into  the  boiler  furnace.  When  the  steam  generated  by  the  boiler  is 
directly  used  for  heating,  a  very  much  greater  proportion  of  the  heat 
in  the  coal  is  converted  to  a  useful  purpose;  in  fact,  this  proportion 
may  be  so  great  as  to  lie  between  60  and  80  per  cent. 

QUESTIONS 

1.  What  is  electroplating  ? 

2.  What  metals  are  generally  used  in  plating  ? 

3.  What  is  the  history  of  electrometallurgy  ? 

4.  What  is  the  salt  of  a  metal  ? 

5.  What  is  nitrate  of  silver  ?     Cyanide  of  silver  ?     Cyanide  of  potassium  ? 

6.  How  may  nitrate  of  silver  be  made  ? 

7.  What  kind  of  vats  are  usually  employed  in  silver  plating  ? 

8.  Why  must  the  anodes  in  a  plating  vat  be  of  the  same  metal  as  the  plate  which 
is  deposited  ? 


FIG.  342.  —  Electric  Smoothing  Iron.  A 
is  point  of  attachment  of  the  conduct- 
ing cord  to  the  electric  circuit. 


ELECTRIC   HEATING   AND   COOKING 


453 


9-    Why  is  nitrate  of  silver  not  used  for  silver  plating  solutions  ? 
10.    What  is  the  best  solution  for  silver  plating  ? 
n.    How  may  cyanide  of  silver  be  made  ? 

12.    Upon  what  does  the  quality  of  the  electrolytic  deposit  of  a  metal  depend  ? 
13-    What  is  the  effect  upon  a  silver  deposit  when  the  current  is  too  great  and 
when  it  is  too  small  ? 

14.  How  are  articles  cleaned  before  plating  ? 

15.  How  are  the  insides  of  silver  articles  gilded  ? 


FlG-   343-— Electric  Kitchen  Outfit. 

1 6.    Upon  what  base  metals  is  nickel  usually  plated  ? 

17-    Why  is  nickel  used  instead  of  silver  for  plating  articles  which  will  receive 
hard  usage  ? 

1 8.  What  solution  is  used  for  nickel  plating  ? 

19.  What  effect  on  nickel  plating  is  caused  by  the  presence  of  organic  impurities 
in  the  bath  ? 

20.  For  what  purpose  is  electrotyping  used  ? 

21.  Why  are  electrotype  moulds  plumbagoed  ? 


454  ELECTRICITY   AND    MAGNETISM 

22.  How  is  an  electrotype  finished  after  the  copper  shell  has  been  deposited  ? 

23.  How  thick  should  the  copper  shell  of  an  electrotype  be  ? 

24.  What  are  the  advantages  of  electrolytic  refining  of  copper  ? 

25.  What  solution  is  used  in  electrolytic  copper  refining  ?    What  is  crude  copper  ? 

26.  What  materials  may  be  looked  upon  as  crude  ores  of  aluminum  ? 

27.  What  is  the  process  for  refining  aluminum  ? 

28.  When  was  electric  smelting  by  the  arc  first  done  ? 

29.  How  is  calcium  carbide  made  ? 

30.  What  is  carborundum  ? 

31.  For  what  kinds  of  metal  working  are  electric  forging  methods  used  ? 

32.  What  is  the  most   common  method    of   electric    forging   and  what   are  its 
advantages  ? 

33.  Describe  the  Thomson  welder. 

34.  What  metals  may  be  welded  by  the  Thomson  method  ? 

35.  How  is  it  that  rings  may  be  welded  by  a  Thomson  welder  ? 

36.  What  is  the  Bernardos  process  of  working  metals  ?     What  is  it  used  for  ? 

37.  What  is  a  pail  welder  ? 

38.  What  happens  in  a  pail  welder  when  it  is  working  ? 

39.  How  is  the  heating  effect  of  the  electric  current  used  for  warming  and  cooking  ? 

40.  Why  cannot  electric  heaters  at  present  replace  stoves  ? 


CHAPTER   XXIII 


ELECTROMAGNETIC   WAVES  ;    WIRELESS  TELEGRAPHY  ; 
ROENTGEN    RAYS 

394.  Water  Waves  or  Vibrations.  —  When  a  pebble  is  thrown  into  a 
pond,  waves  of  water  ripple  away  in  ever  widening  circles,  and  the  waves 
travel  outward  in  all  directions  along  the  surface  of  the  water  from  the 
place  where  the  pebble  strikes.  The  crest  of  each  wave  travels  onward 
until  it  is  either  broken  upon  some  obstruction,  as  the  pond  banks,  or 
is  dissipated  by  the  friction  of  the  water  itself,  when  its  height  becomes 
so  small  as  to  be  no  longer  dis- 
cernible. The  distance  between 
the  crests  of  two  adjacent  waves 
is  called  the  Wave  Length,  while 
the  difference  in  level  between  the 
crest  and  trough  is  called  the  Am- 


FlG.  344.  —  Illustration  of  Amplitude  and 
Wave  Length  of  Waves  on  the  Sur- 
face of  Water. 


plitude  of  the  wave  (Fig.  344). 

If  a  log  lies  in  the   pond,   the 
waves  will  break  upon  it  and  leave 

comparatively  still  water  on  its  farther  side  (thus  creating  what  may  be 
called  a  water-wave  shadow),  though  the  waves  which  pass  the  ends  of 
the  log  will  tend  to  spread  out  and  fill  the  shadow  space. 

If  a  second  set  of  waves  is  created,  they  may  interfere  with  the  motion 
of  the  waves  of  the  first  set  by  striking  against  them,  and  the  waves  of 
the  two  sets  then  become  entangled  and  broken  up.  This  is  called 
Interference  of  the  waves.  However,  if  the  water  is  struck  at  regular 
intervals  by  a  rod  or  a  paddle,  a  certain  rate  of  strokes  may  be  found 
which  will  produce  a  series  of  waves  which  do  not  interfere,  but  which 
move  off  regularly  and  strongly.  It  will  also  be  found  that  the  wave 
length  is  dependent  upon  the  force  and  character  of  the  stroke. 

455 


456  ELECTRICITY   AND    MAGNETISM 

Although  these  water  waves  move  out  rapidly  from  the  centre  of  dis- 
turbance, the  particles  of  water  merely  move  up  and  down,  and  do  not 
flow  away  from  their  position  in  the  pond.  That  is,  the  water  is  set  to 
Vibrating  (or  swinging  up  and  down),  but  not  to  flowing.  This  may 
be  proved  by  observing  a  chip  or  leaf  which  has  been  thrown  into  the 
pond,  when  it  will  be  seen  that  the  float  merely  vibrates  up  and  down 
as  the  waves  pass  it,  and  (if  the  wind  does  not  interfere  with  it)  it  does 
not  change  its  position  in  the  pond,  as  it  necessarily  would  if  the  water 
actually  flowed  along  with  the  waves. 

When  the  vibration  of  the  particles  of  a  body  is  at  right  angles  to  or 
across  the  direction  of  the  movement  of  the  waves,  as -is  the  case  with 
the  water  waves,  it  is  called  Transverse  Vibration.    If,  on  the  other  hand, 
the  panicles  vibrate  along  the  line  of  the  waves  instead  of  across  it,  the 
vibration  is  said  to   be  Longitudinal.     The  vibrations  of  a  stiff  spiral 
spring,  which  has  been  compressed  endwise  and 
then  released,  are   longitudinal  vibrations.     Fig- 
ure 345  illustrates  a  spring  which  is  designed  to 
be  struck  by  a  hammer.    When  the  spring  is  struck, 
waves  of  compression  pass  very  rapidly  from  the 
top  to   the   bottom  of  the  spring,  while  any  part 
of  the  spring  will  merely  vibrate  back  and  forth. 
The  waves  in  the   air   that  we  call  Sound  Waves 
FIG.  345.  —  Spiral  Spring    are  waves  in  which  the  vibration  of  the  particles 
which  may  be  struck    [s  longitudinal.    When  one  utters  a  sound,  spheri- 

by   Hammer  to   illus-          .     .  /-      i  i 

trate  Longitudinal  Vi-    cal-shaped  waves  of  alternately  compressed  and 
bration.  rarefied  air  radiate  away  from  the  speaker  (as  the 

waves  of  water  radiate  from  a  point  of  disturb- 
ance), but  the  air  particles  vibrate  back  and  forth,  longitudinally,  in 
lines  parallel  to  those  of  the  waves  they  create. 

395.  Ether  Waves  or  Radiation.  —  Waves  that  can  be  seen  and 
heard  in  the  ordinary  way  have  the  same  wave  characteristics  as  those 
that  are  supposed  to  be  set  up  in  the  Ether  when  light,  electrical  energy, 
or  magnetic  disturbances  are  transmitted.  We  cannot  conceive  of  light 
and  heat  coming  from  the  sun  without  some  medium  for  transmitting 
them.  The  air  is  supposed  to  extend  only  a  few  miles  beyond  the 
earth's  surface,  and  the  sun  is  millions  of  miles  away,  so  that  the  air 


ELECTROMAGNETIC   WAVES 


457 


cannot  be  the  required  medium.  It  is  believed,  therefore,  that  there  is 
some  other  substance  which  serves  for  the  transmitting  medium.  This 
substance  is  called  Ether.1 

The  ether  is  not  supposed  to  be  matter,  as  we  usually  consider  it ;  but 
is  assumed  to  have  the  capability  of  being  set  into  vibrations  or  wave 
motions,  much  as  a  plate  of  jelly  may  be  caused  to  vibrate  by  gently 
striking  it  at  any  point.  The  ether  is  thought  to  pervade  everything  and 
to  be  everywhere.  Ordinary  matter,  such  as  our  bodies,  is  supposed  to 
be  made  up  of  separated  molecules  or  atoms,  and  the  ether  is  all  about, 
between,  and  perhaps  through  these  particles.  When  we  move  about 
in  ether  we  do  not  disturb  it.  We  move  through  it,  as  the  ghost  of 
the  story  books  is  supposed  to  pass  through  walls  and  other  obstructions 
that  are  impervious  to  us. 

Although  materials  can  be  moved  from  one  place  to  another  without 
disturbing  the  ether,  yet  it  may  be  set  vibrating  if  an  object  is  heated. 
When- a  body  is  heated,  its  particles  are  thought  to  be  set  into  rapid  vibra- 
tion, and  this  sets  up  corresponding  waves  in  the  surrounding  air  and 
ether.  When  you  hold  your  hand  toward  a  hot  object,  your  hand  feels  the 
warmth.  This  feeling  is  caused  by  the  mechanical  heat  waves  in  the  air 
striking  against  your  hand.  If  the  body  is  heated  until  it  is  white  hot, 
ether  waves  of  shorter  lengths  are  generated  by  the  vibrating  particles, 
and  these  cause  the  sensation  of  light  when  they  strike  against  the  retina 
of  the  eye. 

Ether  waves  can  also  be  generated  by  electric  or  magnetic  disturbances. 
When  these  waves  strike  conductors  at  a  distance,  electrical  activity  is 
set  up  in  them.     This  is  now  believed 
to  be  the  medium  of  electromagnetic 
induction.2 

A  very  crude  mechanical  analogy  of 
electromagnetic  induction  may  be  con- 
structed by  sticking  two  pins  in  a  plate 
of  jelly  (Fig.  346).  Now,  if  one  pin 

is  struck  and  caused  to  vibrate,  a  wave  is  set  up  in  the  jelly  which  starts 
the  other  pin  to  vibrating  in  the  same  way.     In  this  way  mechanical 
energy  is  transmitted  by  the  jelly  waves  from  one  pin  to  the  other. 
i  Article  2.  2  Article  132, 


FlG.  346.  —  Jelly  Analogue  of  Electro- 
magnetic Induction. 


458  ELECTRICITY   AND   MAGNETISM 

Or  we  may  compare  the  effect  of  the  ether  waves  to  the  sound  waves 
which  may  be  set  up  in  the  air.  When  a  person  speaks,  waves  are  set 
up  in  the  air  which  strike  upon  the  ear  drum,  causing  it  to  vibrate  ;  thus, 
in  turn,  exciting  the  nerves  of  hearing.  Again,  if  one  floating  block  is 
caused  to  swash  up  and  down  in  the  water  so  as  to  set  up  waves  in  the 
pond,  another  float  in  an  adjoining  part  of  the  pond  receives  an  up  and 
down  motion  when  the  waves  reach  it. 

The  ether  waves  vary  in  frequency  from  several  thousand  trillion 
periods  per  second  to,  possibly,  one  hundred  million  periods  per  second, 
or  less ;  and  the  wave  lengths  vary  from  a  few  millionths  of  an  inch  in 
length  (from  crest  to  crest)  to  several  feet  or  yards  in  length.  The 
waves  all  travel,  so  far  as  is  known  from  experimental  investigation,  at  a 
speed  of  about  186,000  miles  per  second.  This  distance  is  almost  seven 
and  a  half  times  the  circumference  of  the  earth.  The  ether  waves  of 
shortest  known  wave  length  can  be  detected  only  by  the  chemical 
action  they  produce  when  they  strike  upon  certain  substances ;  some 
longer  waves  are  perceived  by  the  eye,  as  light ;  still  longer  ones  become 
apparent  from  their  heating  effect ;  and  the  longest  create  the  electro- 
magnetic phenomena  with  which  we  are  to  deal  in  the  following  articles. 

396.  Electromagnetic  Waves.  —  In  1864,  Clerk  Maxwell,  one  of  the 
most  gifted  mathematicians  of  the  world's  history,  showed  by  a  brilliant 
mathematical  demonstration  that  it  ought  to  be  possible  to  create  ether 
waves  by  electrical  disturbances.1  The  experimental  proof  of  Maxwell's 
mathematics,  however,  was  not  forthcoming  until  1888,  when  Heinrich 
Hertz,  a  German  investigator,  actually  produced  such  waves  by  an  equally 
brilliant  investigation,  but  this  time  carried  on  in  the  laboratory.  He 
created  electromagnetic  waves  by  passing  sparks  from  an  induction  coil 
through  a  gap  between  two  polished  knobs.  An  apparatus  for  the 
purpose,  which  is  called  an  Oscillator,  is  illustrated  in  Figure  347.  In 

1  It  must  be  remembered  that  the  characteristics  of  the  ether,  and  even  its  existence  in 
the  particular  form  in  which  we  conceive  it,  are  purely  theoretical  hypotheses,  the  truth  of 
which  has  not  yet  been  proved.  But  experiments  relating  to  electromagnetic  radiation 
and  allied  phenomena  go  far  toward  justifying  our  acceptance  of  the  ether  theories.  In 
any  event,  these  theories  enable  us  to  gain  a  reasonably  clear  physical  conception  of 
various  occurrences,  such  as  those  described  in  this  chapter,  which  we  might  not  otherwise 
be  able  to  grasp ;  and  the-  experimentally  demonstrated  facts  will  always  be  ours,  even  if 
the  particular  characteristics  which  we  now  ascribe  to  the  ether  are  partly  swept  away  by 
the  advance  of  experimental  science. 


ELECTROMAGNETIC   WAVES 


459 


FIG.  347.  —  Transmitter  for  Wireless  Telegraphy. 
A,  Induction  Coil;  B,  Spark  Gap;  C,  C  Con- 
densers ;  D,  Telegraph  Key. 


this  figure,  A  is  an  induction  coil  for  getting  a  high  pressure,  and  B  is  the 
spark  gap. 

When  a  spark  passes  across  the  gap,  B,  it  looks  like  a  single  flash, 
but  in  reality  the  electric  discharge  flies  back  and  forth  across  the  gap 
many  times  with  inconceiv- 
able rapidity  (possibly  at  the 
rate  of  one  hundred  million 
times    per   second,   or    even 
more).     This  makes  a  rap- 
idly vibrating  or  alternating 
current  which  gradually  dies 
out,  as  illustrated  in  Figure 

348.  The  effect  is  analogous 
to  the  mechanical  action  of 
a    spiral    spring    which    has 
been   compressed    and   then 

released.  When  the  balls  are  charged  to  a  high  difference  of  pressure, 
we  may  consider  that  the  medium  surrounding  the  balls,  which  is  called 
the  dielectric,1  i.i  under  an  electrical  pressure  or  stress ;  and  the  pass- 
ing of  the  rapidly  vibrating  spark  occurs  when  the  dielectric  is  relieved 

by  the  spark  breaking  through,  and  the 
electric  current  surges  back  and  forth  like 
the  spiral  spring  when  it  is  released. 

Now  these  vibrations  of  electricity,  which 
really  pass  through  the  entire  apparatus, 
are  capable  of  setting  up  waves  in  the  ether 
which  pass  out  in  all  directions,  but  more 
strongly  in  a  direction  at  right  angles  to 
the  spark  gap  and  the  wings  CC  (Fig. 
347).  The  ether  waves  which  are  created 
in  this  manner  are  called  Electromagnetic 
Waves.  To  detect  these  waves,  Hertz  used 
a  ring  of  wire  containing  a  small  spark  gap,  such  as  is  shown  in  Figure 

349.  When  this  Resonator,  as  it  is  called,  is  held  so  that  the  ether 
waves  pass  through  the  ring,  electrical  vibrations  are  set  up  in  the  ring 

1  Article  28. 


WAVE  LENGTH 


FIG.  348.  — Chart   of  Oscillatory 
Electric  Discharge. 


460  ELECTRICITY  AND   MAGNETISM 

and  are  indicated  by  small  sparks  passing  across  the  air  gap.  The 
effect  of  the  oscillator  on  the  resonator  may  be  compared  to  two  tuning 

forks  of  the  same  pitch  which  are  placed 
at  a  distance  apart.  When  one  fork  is 
set  to  vibrating,  the  air  or  sound  waves 
beat  upon  the  other  and  set  it  also  to 
vibrating.  This  may  be  perceived  by 
carefully  examining  the  second  fork  a 
short  time  after  the  first  fork  is  set  to 
vibrating,  when  it  may  be  readily  noticed 
that  the  second  fork  is  vibrating. 

It  is  necessary  to  have  the  two  forks 
FIG.  349.— Hertzian  Resonator  or    of  the   same   tone  or   musical  note    to 

Ether  Wave  Indicator.     /?,  Spark      get.  thjs  effect   m   ^  fu]]est   pQwer<      The 

second  fork  vibrates  because  the  sound 

waves  beat  upon  it  at  a  rate  which  is  equal  to  the  natural  rate  of  vibra- 
tion of  the  fork,  and  each  impulse  from  the  sound  waves  adds  to  the 
preceding  impulses.  And  just  so  it  is  necessary  to  have  the  electric 
oscillator  and  resonator  tuned  together,  so  that  the  resonator  may 
naturally  respond  to  the  ether  waves  projected  upon  it  by  the  oscillator 
in  a  manner  that  is  analogous  to  that  with  which  the  second  tuning  fork 
responds  to  the  sound  waves  projected  upon  it  by  the  first  fork.  If  one 
swings  in  a  hammock,  it  is  desirable  to  give  the  pushes  which  cause  the 
hammock  to  swing  (vibrate)  at  the  same  rate  at  which  the  hammock  is 
going  back  and  forth.  If  the  pushes  are  given  at  any  other  rate,  they 
tend  at  intervals  to  stop  the  motion  and  at  other  intervals  to  help  it ; 
that  is,  there  is  interference. 

The  natural  rate  of  electrical  vibration  of  the  resonator  or  oscillator  is 
dependent  upon  the  dimensions  of  the  conductor  composing  the  instru- 
ment, and  an  adjustment  may  be  effected  by  changing  the  sizes  of  the 
wings,  CC,  on  the  oscillator  (Fig.  347),  or  the  resonator  ring  (Fig.  349), 
until  their  rates  or  frequency  are  the  same. 

It  has  been  learned  by  experiment  that  the  electromagnetic  ether 
waves  can  be  Reflected,  Refracted,  Polarized,  etc.,  just  as  can  light  waves 
or  heat  waves,  which  clearly  indicates  the  close  alliance  of  the  different 
kinds  of  radiation. 


WIRELESS  TELEGRAPHY 


461 


FIG.  350. — A  Coherer  by  Means  of  which 
Ether  Waves  may  be  detected. 


397.  Wireless  Telegraphy.  —  The  ether  waves  may  also  be  detected 
by  what  is  called  a  Coherer  (Fig.  350).  In  the  figure,  A  is  a  small 
glass  tube  with  metal  rods  projecting  into  it  at  either  end,  but  between 
the  ends  of  which  there  is  a  small 
space.  This  space  at  the  centre  is 
partly  filled  with  metal  filings,  prefer- 
ably of  silver.  The  electrical  resist- 
ance through  these  filings  is  ordinarily 
several  thousand  ohms,  but  when  prop- 
erly tuned  ether  waves  from  an  oscil- 
lator strike  upon  the  tube,  the  filings 
Cohere,  or  stick  together,  thus  reduc- 
ing the  resistance  to  a  very  few  ohms.  The  cause  of  the  cohering  is 
supposed  to  be  due  to  the  particles  of  .the  filings  being  drawn  together 
by  electrostatic  or  electromagnetic  attractions  set  up  between  them. 

If  the  circuit  of  a  battery  (composed  of  a  cell  or  two)  and  a  relay1  is 
completed  through  the  coherer  by  making  connections  to  the  two  metal 

rods  at  the  ends  of 
the  coherer,  the  relay 
may  be  so  adjusted 
that  it  will  only  work 
when  the  resistance 
of  the  circuit  is  low- 
ered by  the  coher- 
ence of  the  filings. 
If  the  relay  is  made 
to  work  a  local  circuit 
containing  a  sounder,2 
this  sounder  will  click 
whenever  the  proper 
waves  from  an  oscil- 

FlG.   351.  —  Diagram   of  Wireless  Telegraph   Receiving 

Station.  lator    strike    the    co- 

herer. 

After  the  filings  have  once  cohered  they  will  remain  so  unless  they 
are  shaken  apart ;  therefore,  a  second  electromagnet  is  usually  placed  in 
1  Article  299.  2  Article  298. 


462 


ELECTRICITY   AND    MAGNETISM 


the  local  circuit  which  contains  the  sounder,  and  this  magnet  is  so 
arranged  that  whenever  a  current  passes  through  its  coils  its  armature  is 
caused  to  strike  against  the  glass  tube,  and  thus  Decohere  the  filings  by 
giving  them  a  sudden  jar.  Figure  35 1  shows  the  connections  of  the 
circuit.  Now,  if  a  key  is  used  for  opening  and  closing  the  primary 
circuit  of  the  induction  coil  in  an  oscillator  (as  shown  at  D  in  Figure 


FIG.  352.  —  Receiving  Apparatus  for  Wireless  Telegraphy. 

347),  telegraph  signals  may  be  transmitted  by  means  of  the  ether  waves, 
which  may  be  received  and  recorded  by  a  receiving  apparatus  like  that 
just  described.  Telegraphing  by  such  means  is  usually  called  Wireless 
Telegraphy,  since  the  signals  traverse  space  by  means  of  the  ether  waves 
and  without  the  intervention  of  wires.  Figure  347  shows  a  commercial 
transmitter  for  wireless  telegraphy,  and  Figure  352  shows  a  suitable 


RECEIVING  STATION 


TRANSMITTING  STATION 


1/1G-  353-  —  Diagramatic  Illustration  of  Transmitting  and  Receiving  Stations  for  Wireless 

Telegraphy. 

arrangement  of  the  receiving  apparatus.     Figure  353  is  a  diagram  of 
receiving  and  transmitting  stations  ready  for  operation. 

Where  the  distance  between  the  transmitting  and  receiving  stations 
is  great,  one  terminal  of  the  coherer  and  one  of  the  oscillator  are  con- 
nected to  earth,  and  the  other  respective  terminals  are  connected  to 
wires  that  run  a  hundred  or  more  feet  vertically  into  the  air.  A  light 


CATHODE   RAYS  463 

plate  of  metal  is  usually  attached  to  the  upper  end  of  each  of  these 
wires. 

William  Marconi,  an  Italian,  has  succeeded  in  transmitting  messages 
a  distance  of  sixty  miles  by  the  means  described.  The  signals  can  be 
transmitted  through  hills  and  walls  with  success,  for  the  materials  com- 
prising them  are  transparent  to  the  electromagnetic  ether  waves,  just  as 
glass  and  mica  are  transparent  to  light  waves. 

398.  Cathode  Rays.  —  There  are  many  other  phenomena  which  are 
supposed  to  be  caused  by  waves  or  disturbances  in  the  ether,  and  which 
may  be  created  through  electrical  means.  Professor  Hittorf,  of  Germany, 
(in  1869),  and  later,  Professor  William  Crookes  (1874-1879),  a  cele- 
brated English  chemist,  made  valuable  investigations  on  the  effect  of 
electric  discharges  through  a  vacuum. 

If  a  closed  tube  or  bulb  of  glass  has  two  wire  terminals,  or  electrodes, 
sealed  into  the  glass  and  protruding  within  the  tube  in  the  way  that  is 
illustrated  in  Figure  354,  a  curious  flickering  light  or  glow  is  observed 
within  the  tube  when  the  electrodes  are  attached  to  the  terminals  of  an 
induction  coil,  provided  the  air 
has  been  previously  exhausted 
from  the  tube  after  the  manner 
of  exhausting  applied  to  incail-  FIG.  354.  — Simple  Form  of  Crookes  Tube, 
descent  lamps.1  If  the  vacuum 

is  poor,  the  light  produced  is  of  an  even,  delicate  color  through- 
out. If  the  vacuum  is  improved,  the  light  near  the  terminal  from 
which  the  current  of  the  discharge  leaves,  which  is  called  the  cathode, 
becomes  somewhat  violet  in  color,  while  the  remainder  of  the  tube  up 
to  the  other  terminal,  or  anode,  is  filled  with  beautiful  masses  of  chang- 
ing and  varying  light.  Crookes  found  that  the  glass  of  the  tube  itself 
apparently  lights  up  by  a  peculiar,  delicate  light  when  the  tube  is  very 
highly  exhausted,  —  the  color  of  the  light  depending  upon  the  kind  of 
glass  (this  state  of  the  glass  is  called  Fluorescence),  —  and  then  but  little 
light  is  seen  in  the  space  within  the  tube,  except  for  occasional  flashes. 
It  is  supposed  that  these  effects  are  the  result  of  a  phenomenon  called 
Cathode  Rays. 

Crookes  found  that  if  the  cathode  of  the  tube  is  made  in  the  shape 

i  Article  256. 


464 


ELECTRICITY   AND    MAGNETISM 


of  a  concave  mirror,  the  rays  meet  at  a  point,  and  that  they  can  be 
caused  to  heat  a  piece  of  metal  located  at  the  point  to  a  white  heat. 

These  rays  apparently  cannot  be  induced  to  come  through  the  glass 
walls  of  the  tube,  but  Dr.  Philip  Lenard  in  1893,  following  a  suggestion 
of  Hertz,  induced  them  to  pass  through  an  aluminum  window  which  he 
inserted  in  the  glass  walls.  Lenard  discovered  that  these  extensions  of 
the  cathode  rays  act  upon  a  photographic  film,  and  in  other  respects 
perform  in  a  most  remarkable  manner. 

It  is  now  thought  that  the  cathode  rays  consist  of  very  minute  flying 
particles  of  matter  —  much  smaller  particles  than  those  we  ordinarily 
conceive  as  atoms.  These  are  presumably  shot  off  from  the  cathode 
by  some  unknown  electrical  action,  and  travel  away  with  a  speed  that 
is  perhaps  one-twentieth  as  great  as  the  velocity  of  light.  The  flying 
particles,  or  "  corpuscles,"  as  they  are  named,  carry  negative  charges 
of  electricity.  The  explanation  of  their  production  and  character  is 
still  very  obscure  and  must  remain  little  better  than  a  happy  guess  until 
additional  experimental  knowledge  of  them  is  obtained. 

399.  Roentgen  Rays.  —  William  Konrad  Roentgen  took  up  the  study 
of  cathode  rays  in  the  fall  of  the  year  1895.  He  then  held  the  chair  of 

physics  in  the  University  of 
Wiirtzburg  in  Bavaria,  and 
was  well  known  in  Germany 
as  an  original  experimenter 
in  physical  science.  On 
Nov.  8  of  that  year  he  was 
experimenting  with  a  well- 
exhausted  Crookes  tube 
(Fig.  354)  which  was  cov- 
ered by  black  cardboard,  so 
that  no  ordinary  light  could 
pass  from  it  to  the  room. 
Near  by  lay  a  sheet  of  paper 
covered  with  a  chemical  com- 
pound which  shines  when 
Crystals  of  the  tungstate  of 


FIG.  355.  — Typical  X-Ray  Tube,     a,  Cathode ;  b,  c, 
Anodes ;  d,  Metal  Source  of  X-Rays ;  X,  X-Rays. 


struck  by  ether  waves  of  high  frequency. 


calcium  possess  this  property,  which  is  called  Fluorescence.     Professor 


ROENTGEN  RAYS 


465 


Roentgen  noticed  a  peculiar  line  occurring  on  this  paper  while  the  tube 
was  working,  which  indicated  that  something  like  light  proceeded  from 
the  tube  and  cast  a  shadow  upon  the  paper.  An  investigation  showed 
that  the  effect  was  due  to  an  hitherto  unknown  radiation  proceeding 
from  the  tube,  and  Roentgen  or  X  Rays  were  discovered. 

These  rays  or  waves  of  Roentgen  are  apparently  created  where  the 
cathode  rays  of  a  Crookes 
tube  strike  a  solid  object 
like  the  glass  walls  of  the 
tube.  If  the  cathode  rays 
are  focussed  upon  a  bit  of 
metal  by  a  concave  cath- 
ode, the  Roentgen  waves 
may  radiate  from  the  sur- 
face of  the  metal.  Figure 
355  shows  diagrammati- 
cally  the  cathode  rays  fo- 
cussed upon  such  a  metal 
piece,  and  the  X-rays,  X, 
passing  downward.  The 
tube  shown  in  the  figure 
is  of  typical  form,  though 
they  are  now  made  of  va- 
rious sizes  and  shapes. 

Roentgen     found     that 

different     materials     held 

FlG.  356.  —  Radiograph  of  Hand  with  Gold  Ring  on 

between  the  working  tube  Third  Finger, 

and   a   fluorescent    screen 

—  now  called  a  fluoroscope  —  greatly  differ  in  their  transparency  to 
X-rays.  Heavy  (that  is,  dense)  metals  or  materials,  such  as  zinc  or 
iron,  in  general  cut  off  the  X-rays  to  a  large  extent,  and  thus  cast  a 
shadow  on  the  fluoroscope,  while  light  materials,  like  wood  and  alumi- 
num, seem  to  be  transparent  to  the  rays,  and  to  cast  almost  no  shadow. 
When  a  hand  is  held  between  a  fluoroscope  and  a  tube,  the  denser 
bones  cast  a  distinct  shadow,  and  the  flesh  casts  scarcely  any.  In  the 
same  way,  silver  money  in  a  pocket-book  casts  a  dense  shadow,  while 
2  H 


466 


ELECTRICITY   AND    MAGNETISM 


the  leather  book  casts  very  little ;  and  nails  driven  into  a  wooden  block 
make  a  black  shadow,  while  the  wood  is  light. 

Having  investigated  the  rays  by  the  use  of  a  screen,  Roentgen  tried 
a  photographic  plate  and  found  that  the  rays  affected  it  in  the  same 
manner  as  light.  Therefore,  when  the  hand  is  placed  between  the  tube 
and  the  plate,  a  "  radiograph  "  is  taken  of  the  bones,  the  less  dense 
flesh  parts  showing  very  indistinctly,  while  the  bones  are  clearly  out- 
lined. Figure  356  shows  a 
"  radiograph  "  of  a  hand, 
and  Figure  357  shows  one 
of  a  rat.  The  rays  are 
nearly  all  stopped  in  their 
progress  from  the  X-ray 
tube  to  the  photographic 
plate  by  the  denser  bones, 
but  the  flesh  is  nearly  trans- 
parent to  the  rays.  Conse- 
quently, the  photographic 
negative  gives  almost  the 
appearance  of  a  negative 
which  shows  a  skeleton 
hand,  or  a  skeleton  rat,  or 
other  body.  If  the  sensi- 
tive photographic  plate  is 
bought  in  paper  wrappings, 
it  is  not  necessary  to  re- 
move them,  as  they  are 
easily  pierced  by  the  rays. 
Figure  358  shows  the  com- 
plete apparatus  for  taking 
X-ray  pictures  which  is  used 

in  hospitals,  with  its  battery,  induction  coil,  tube,  plate,  and  operating 
chair.  The  patient  is  about  to  have  a  radiograph  taken  of  the  interior 
of  his  chest.  Such  photographs  are  taken  for  the  purpose  of  locating 
fractures  in  the  bones,  extraneous  metal  objects,  like  bullets  or  needles 
in  the  flesh,  and  other  similar  purposes. 


FIG.  357.  —  Radiograph  of  a  Rat. 


THE   ADVANCEMENT   OF   KNOWLEDGE 


467 


The  true  nature  of  Roentgen  rays  has  yet  to  be  determined,  but  it  is 
competently  suggested  that  they  consist  of  waves  or  ripples,  of  very  short 
wave  length,  started  in  the  ether  when  the  corpuscles  of  cathode  rays 
collide  with  a  solid  object.  We  must,  however,  depend  on  the  illumi- 
nating discoveries  of 
the  future  to  enable 
us  to  safely  draw  con- 
clusions regarding  the 
exact  relations  of 
these  ether  ripples  to 
the  ether  waves  called 
light,  or  the  other 
ether  manifestations 
called  electricity  and 
magnetism. 

The  lines  of  investi- 
gation considered  in 
this  chapter  seem  to 
be  of  narrow  breadth, 
but  they  are  appar- 
ently just  touching  the 

great    field    of   kliowl-        F^  ^  _  Radiographic  Apparatus  arranged  for  Use  in 

edge    that    is    to    be  a  Hospital, 

ultimately  opened   to 

us,  step  by  step,  and  which  will  perhaps  cause  the  greatest  industrial 
advances.  The  men  whose  names  have  appeared  most  frequently  in 
this  book — Gibert,  Franklin,  Ohm,  Ampere,  Volta,  Faraday,  Oersted, 
Davy,  Henry,  Maxwell,  Crookes,  Hertz,  Roentgen,  Kelvin,  Siemens, 
Gramme  —  and  many  others,  have  each  added  a  portion  to  the  ever 
growing  sum  of  man's  knowledge  of  physical  facts  and  laws.  The  ad- 
vancement is  steadily  proceeding,  and  will  surely  continue  to  proceed. 
New  discoveries  based  upon  those  grown  old  are  continually  recorded 
by  the  great  band  of  investigators  of  the  world.  We  know  little  of  the 
laws  of  the  universe,  and  much  less  of  its  fundamental  structure.  The 
study  of  electromagnetic  phenomena  seems  to  lead  toward  an  unravel- 
ling of  the  unknown  in  nature.  If  we  ever  learn  the  true  constitution 


468  ELECTRICITY  AND   MAGNETISM 

of  electric  and  magnetic  phenomena,  \ve  may  expect  at  the  same  instant 
to  know  the  constitution  of  matter  and  the  truth  regarding  the  hypo- 
thetical ether.  With  such  knowledge,  the  character  of  man's  life  may 
enter  a  condition  of  satisfaction  and  convenience  exceeding  our  richest 
dreams.  Many  of  our  common  mechanical  necessities  (the  telegraph, 
the  telephone,  the  electric  light,  the  electric  car)  were  lately  almost 
inconceivable,  and  the  adaptation  of  present  known  forces  to  man's  use 
is  only  begun.  New  discoveries  are  also  gradually  bringing  to  us  the 
possibility  of  new  utilities  of  which  we  do  not  now  conceive.  And  the 
twentieth  century  may  be  expected  to  exert  the  most  beneficent  influ- 
ence on  civilization  through  man's  more  intelligent  and  perfect  applica- 
tion of  nature's  laws. 

QUESTIONS 

1.  What  is  meant  by  the  phrase,  wave  length  ?     The  phrase,  amplitude  of  a  wave  ? 

2.  When  a  pebble  is  thrown  into  a  pond,  does  the  water  move  with  the  waves  ? 

3.  What  is  transverse  vibration  ?     Longitudinal  vibration  ? 

4.  Give  examples  of  transverse  and  longitudinal  vibrations. 

5.  How  is  light  transmitted  ? 

6.  How  can  a  piece  of  coal  be  made  to  create  ether  waves  ? 

7.  Give  mechanical  and  sound  analogies  to  transmission  of  light  and  heat  by 
ether  waves. 

8.  How  fast  do  ether  waves  travel  ? 

9.  How  long  are  ether  waves  ?     What  is  their  frequency  ? 

10.  What  did  Maxwell  point  out  in  1864  ?     When  did  Hertz  make  his  discovery  ? 

11.  Give  a  mechanical  analogy  to  an  electric  spark. 

12.  What  is  an  oscillator  ?     A  resonator  ?     How  did  Hertz  use  them  ? 

13.  Why  must  an  oscillator  and  resonator  be  tuned  to  work  together  ?     Give  a 
mechanical  analogue. 

14.  What  is  a  coherer  ?     How  does  it  work  ? 

15.  What  is  wireless  telegraphy  ?     Describe  the  apparatus  in  detail  and  state  the 
use  of  each  part. 

1 6.  How  can  you  telegraph  through  the  walls  of  a  building  ?     Why  ? 

17.  What  did  William  Crookes  discover  in  1875  ? 

1 8.  What  happens  when  a  spark  is  passed  through  an  exhausted  tube  ? 

19.  What  are  cathode  rays  ? 

20.  What  are  Roentgen  rays  ?     When  were  Roentgen  rays  discovered  ? 

21.  How  are  Roentgen  rays  obtained  ?     How  are  radiographs  made  ? 

22.  Are  light  or  dense  substances  pierced  the  more  readily  by  Roentgen  rays  ? 

23.  To  what  use  are  Roentgen  rays  put  in  hospitals  and  by  physicians  ? 


INDEX 


Accumulators,  46-47. 

Acid  radical,  52,  53,  431. 

Acids,  relative  conducting  power  of,  6. 

Action,  amount  of  chemical,  in  battery  cell, 
45;  law  of  electrochemical,  45 ;  in  electro- 
lytic cells,  51-53;  theories  of  electrolytic, 
58-61. 

Ageing  of  magnets,  69-70. 

Air,  conducting  powers  of  dry,  6;  specific 
inductive  capacity  of,  204-205 ;  carbon 
vapor  a  better  conductor  than,  273- 
274. 

Air  space  in  armatures,  221. 

Alloys,  conducting  powers  of,  82-83;  tem- 
perature  coefficients  of,  88  ;  in  electro- 
plating, 438-439- 

Alphabet,  Morse  telegraphic,  337,  340. 

Alternating  current,  145-146 ;  transformers, 
150-151 ;  amperemeters,  190-191 ;  obeys 
same  laws  as  direct  current,  236 ;  likened 
to  flow  of  water  in  tideway,  239-240  ;  "  out 
of  phase,"  240;  lag  of,  244,  256-257; 
chemical  effect  of,  246-247  ;  heating  effect 
of,  247-248 ;  measurement  of,  249-253 ; 
frequency  and  period  of,  253;  effect  of 
self-induction  of  flow  of,  253-255  ;  system 
of  electrical  distribution,  394.  See  also 
Current. 

Alternators,  principle  of,  245 ;  construction, 
264-266;  inductor,  266;  in  parallel,  267; 
polyphase,  268-269. 

Alumina,  441. 

Aluminum,  value  of,  as  electrochemical  equiv- 
alent, 56;  as  a  conductor,  83;  wires  of, 
for  light  and  power  lines,  370;  safe  carry- 
ing capacity  of  wires  of,  399 ;  electrical 
production  of,  441-442. 

Amalgamation  of  zinc,  44. 

Amber,  i,  3. 

Ammeter,  see  Amperemeter. 


Ammonium  chloride,  solution  of,  in  battery 
cells,  37 ;  used  in  chloride  of  silver  battery, 
40. 

Ampere,  Andre  Marie,  122,  129,  236,  335; 
theory  of,  concerning  magnetism,  73,  125. 

Ampere,  the,  denned,  22;  the  international, 
87. 

Ampere  hour,  203. 

Ampere  second,  203. 

Ampere  turns,  defined,  124;  relation  between, 
and  magnetism,  130. 

Amperemeter,  180;  uses  of,  183;  mechanism 
of  magnetic,  183-185 ;  the  Weston,  185- 
187;  hot  wire,  189;  alternating  current, 
190-191. 

Amperemeter  scales,  189-190. 

Amplitude  of  wave  defined,  455. 

Animals,  conducting  powers  of,  6 ;  electricity 
existent  in,  113-114. 

Anode  defined,  51. 

Anodes,  positions  of,  and  cathodes,  in  electro-  _ 
plating,  434,  436,  440. 

Arago,  129,  236,  335. 

Arc,  the  electric,  273-275 ;  smelting  by  the, 
442-443. 

Arc  lamps,  mechanism  of,  275-278  ;  enclosed, 
278 ;  double,  279-280 ;  operation  of,  278- 
280;  enclosed,  coming  into  use  for  out- 
door lighting,  426. 

Arc-light  lines,  wire  for,  371 ;  testing,  419-421. 

Arc  process  of  welding,  449-450. 

Armature,  dynamo,  213;  one-coil,  214-215 ; 
the  Gramme,  216-217;  drum,  218-219; 
toothed  or  slotted,  222-223 ;  alternator, 
265 ;  squirrel-cage,  270 ;  effect  of  resist- 
ance of  motor,  323-324;  of  recording 
telegraph  register,  339-340. 

Armor  plate,  softening,  by  electric  heating, 

449- 

Astatic  needles,  159. 
Attraction,  magnetic,  11-12,  65-66 ;  force  of, 


469 


4/0 


INDEX 


between  two  bodies  is  mutual,  77  ;  mutual, 

of  electric  circuits,  154. 
Austral  fluid,  Coulomb's,  72. 
Auiomobile,  electric,  333. 


Ballistic  galvanometer,  208. 

Bar  magnets,  68. 

Base  of  incandescent  lamp,  285-286. 

Battery,  28  ;  connected  in  series,  33 ;  bichro- 
mate, 37-38 ;  gravity,  41-43 ;  Daniell's, 
42;  dry,  43-44;  primary,  45;  storage, 
46-47 ;  secondary,  47 ;  where  valuable, 
46;  the  testing,  177. 

Battery  cell,  see  Cell. 

Bauxite,  441. 

Bearings,  changes  in  compass,  79. 

Bell,  Alexander  Graham,  352. 

Bellows,  organ,  driven  by  electric  motor, 
320,  321.  . 

Bells,  electric,  open  circuit  cells  used  for, 
35 ;  Leclanche  battery  cell  for  ringing, 
40;  wiring,  364-366;  mechanism  of, 
366-367 ;  single-stroke,  367. 

Berlin,  first  practical  electric  railway  at,  308. 

Bernardos,  arc  welding  process  developed 
by,  449. 

Bichromate  battery  described,  37. 

Bichromate  of  potash,  used  in  chemical 
depolarization,  37 ;  nitric  acid  more 
powerful  than,  39. 

Bichromate  of  soda  used  in  chemical  de- 
polarization, 37. 

Birmingham  wire  gauge  (B.W.G.) ,  diameters 
of  wires  drawn  to,  387. 

Bleaching  powder,  chloride  of  lime,  used  in 
chemical  depolarization,  37. 

Blood,  analogy  between  pulsating  currents 
and  flow  of,  238-239. 

Blowing  of  fuse,  405 ;  caused  by  poor  con- 
nections, 419. 

Bluestone,  see  Blue  vitriol. 

Blue  vitriol,  41,  42,  52;  used  in  copper  plat- 
ing, 438. 

Bond  wires  for  electric  railways,  312. 

Boreal  fluid,  Coulomb's,  72. 

Brass  balls  charged  by  induction,  6-8. 

Break  in  electric  line,  410-411. 

Breaker,  automatic  circuit,  398-399. 

Bridge  duplex  telegraph  system,  348 ;  for 
ocean  cables,  351., 


!  Bronze,  aluminum,  442. 

Brown  and  Sharp  (B.  &  S.)  gauge,  charac- 
teristics of  wire  drawn  to,  385-386. 

Brush  arc-light  dynamo,  280-281. 

Brush  Electric  Company,  280. 

Brushes,  dynamo,  213;  proper  position  of, 
233-234. 

Bulb  of  incandescent  lamp,  283-284. 

Bunsen  cell,  38-39. 

Bunsen  photometer,  424. 

Burning  out  of  dynamos,  231. 

Burnishers,  435. 

Bus  bars,  304. 

Cable,  importance  of  capacity  of  submarine, 
26;  history  and  description  oi  submarine, 
349-351;  wires  bunched  into  a,  377;  un- 
derground, 378-382 ;  testing  underground 
and  submarine,  418. 

Calcium  carbide,  443. 

Calibration  curve  of  galvanometer,  161-162. 

Calorie  defined,  109-110. 

Calorimeter,  109-110. 

Canals  of  Niagara  Falls  Power  Company, 

299-300. 
j  Candle  foot  defined,  427. 

Candle  power  defined,  279. 

Capacity,  electrical,  24;  of  condensers,  25- 
26;  specific  inductive,  204-205. 

Car,  electric,  two  motors  on  each,  310. 

Carbon,  used  for  positive  plate  in  bichro- 
mate battery,  37  ;  effect  of  temperature  on 
resistance  of,  87-88 ;  production  of  fila- 
ments of,  284-285 ;  in  microphone,  356- 

357.  363- 

Carbon  dust  caused  by  careless  trimming  of 

arc  lamps,  426. 
j  Carbon  filament  lamp,  283. 

Carbon  rod,  276. 

,  Carbon  vapor  in  arc  lights,  273-275. 
j  Carbonate  of  copper,  51. 
1  Carbons  used  in  arc  lamps,  280. 

Carborundum,  443-444. 

Cardew,  193. 

Cardew  voltmeter  used  in  alternating  cur- 
rent measurements,  252. 

Carhart,  Professor  Henry  S.,  197. 

Carpet,  electrical  charge  gathered  from,  4. 

Carriage,  electric,  333. 

Cascade,  condensers  connected  in,  207. 

Cathode  defined,  51. 


INDEX 


471 


Cathode  rays,  463-464. 

Cathodes,  positions  of,  and  anodes  in  elec- 
troplating, 434,  436,  440. 

Cell,  simple  battery,  28-29;  voltaic,  29-30; 
Gassner's  dry,  43-44 ;  Clark's,  197. 

Cells,  electric  battery,  32 ;  electrical  pressure 
of,  33 '.  connected  in  series,  33 ;  pressure 
of,  independent  of  size,  33-34 ;  polariza- 
tion of,  34 ;  methods  of  depolarizing,  34 ; 
open  circuit,  35,  36,  41 ;  closed  circuit,  35 ; 
Bunsen,  38-39;  Grove,  38-39;  Edison- 
Lalande,  39-40  ;  Leclanche,  40-41 ; 
Daniell's,  41-42;  connected  in  series, 
chemical  action  the  same  in  all,  45; 
electrolytic,  51-53. 

Centimeter  defined,  13. 

Chaperone,  39. 

Charcoal,  relative  conducting  power  of,  6. 

Charge,  electrical,  3. 

Charging  storage  batteries,  process  of,  47. 

Chemical  equivalents  defined,  54. 

Chicago,  electrical  congress  held  at,  86-87; 
first  American  electric  railway  open  to 
public  at,  308. 

Chicago  Electrical  Congress  adopts  Clark's 
cell  as  comparative  standard  of  pressures, 
197. 

Chimes,  electric,  59-60. 

Chloride  of  copper,  51. 

Chloride  of  lime  bleaching  powder  used  in 
chemical  depolarization,  37. 

Chloride  of  silver  battery,  39,  40. 

Circuit,  closed,  32 ;  measurement  of  power  in 
an  alternating,  259-261 ;  cutting  machines 
and  dynamos  out  of,  305-307 ;  local,  in 
telegraph  apparatus,  343 ;  metallic,  for 
telephones,  363,  374 ;  wire  for  bell,  364-366. 

Circuit  breakers,  automatic,  398-399. 

Circuits,  branched,  compound,  and  derived, 
97 ;  mutual  attraction  or  repulsion  of 
electric,  154;  in  series,  92;  in  parallel, 
93-95 ;  divided,  95 ;  series  and  parallel 
combined,  97. 

Clark's  cell,  197. 

Clausius,  theory  of  electrolytic  dissociation 
of,  60-61. 

Cleat  and  moulding  wiring,  401-402. 

Clock  rule  for  relative  direction  of  current 
and  magnetism  in  solenoid,  125. 

Clock-work  arc-lamp  mechanism,  276-277. 

Closed  circuit  cells,  35. 


Clutch  arc-lamp  mechanism,  276-277. 

Coatings  of  condenser,  25. 

Cobalt,  one  of  commonest  magnetic  ma- 
terials, 67. 

Code,  National  Electrical,  400-401. 

Coercive  force,  69. 

Coil,  Ruhmkorff,  150;  spark,  153. 

Coils,  induction,  149;  for  resistance  boxes, 
170;  induction,  in  telephone  transmitter, 

357- 

Collecting  rings  of  dynamo,  213. 

Columbus,  experience  of,  with  magnetic 
needle,  65,  79-80. 

Comb  of  friction  machine,  18-19. 

Combustion,  28. 

Commutator,  213-214. 

Comparison,  measurement  of  pressure  by, 
196-197. 

Compass,  Chinese  said  to  have  used,  63; 
changes  in  bearings  of,  79;  local  varia- 
,tions  of,  79 ;  method  of  determining 
direction  of  current  by,  123. 

Condensers,  24-25 ;  capacity  of,  204-205 ; 
relation  of  pressure,  charge,  and  capacity 
in,  205-206;  standard,  207-208. 

Conduction  of  heat  from  wire,  112. 

Conductivity  of  metals,  6,  82-83 1  specific 
magnetic,  133 ;  measurement  of,  of  me- 
tallic circuit,  412-413. 

Conductor,  static  electricity  remains  on  sur- 
face of,  13-14 ;  water  as  a,  57. 

Conductors,  electrical,  5;  electrolytic,  51; 
specific  resistance  of,  varies,  91 ;  joint 
resistance  of,  in  parallel,  97-98. 

Conduit,  electric,  378-382. 

Conduits  for  inside  wiring,  403. 

Congress,  Electrical,  at  Chicago,  86-87. 

Connections,  parallel  and  series,  for  lamps 
and  motors,  286-287. 

Conservation  of  energy,  law  of,  107. 

Constant  of  galvanometer,  161. 

Construction,  electric  line,  369-382. 

Control,  of  electric  cars,  316-317;  series- 
parallel,  317-319. 

Convection  of  heat  from  wire,  112. 

Converters,  150-151 ;  rotary,  270-271. 

Cooking  by  electricity,  451-452. 

Copper,  commonest  salts  of,  51 ;  value  of,  as 
electrochemical  equivalent,  56;  ranks 
with  silver  as  best  conductor  known, 
82-83 1  temperature  coefficient  of,  88 ; 


4/2 


INDEX 


resistance  of  a  mil  foot  of,  90;  electro- 
chemical equivalent  of,  166;  in  voltam- 
eter, 166;  ior  winding  armatures  and 
fields,  230-231 ;  wires  of,  for  telegraph, 
telephone,  and  light  and  power  lines,  370 ; 
data  concerning  properties  of  wire  of, 
385-387 ;  saving  of,  in  three-wire  and 
five-wire  systems  of  electrical  distribu- 
tion, 394 ;  safe  carrying  capacity  of  con- 
ductors of,  399 ;  electrolytic,  439 ;  refining, 
439-440. 

Copper  oxide  used  as  a  depolarizer,  39-40. 

Copper  plating,  52-53,  437-438. 

Copper  sulphate  battery,  41-43. 

Core  laminations,  219-220. 

Cotton,  attitude  of,  when  electrically  charged, 
toward  other  substances,  5. 

Coulomb,  theories  of,  concerning  magnetism, 
72. 

Coulomb,  the,  12-13 ;  meter,  203. 

Crater  of  electric  arc,  274. 

Crib,  406. 

Crookes,  William,  463. 

Crookes  tube,  463. 

Cross  in  telegraph  or  telephone  line,  410; 
location  of  a,  415-418. 

Cross  arms,  369,  370. 

Cryolite  used  in  reduction  of  aluminum,  441. 

Current,  difference  of  electrical  pressure 
necessary  to  obtain  continuous,  31 ;  flows 
from  point  of  high  pressure  to  that  of  low 
pressure,  33  ;  relation  between,  and  press- 
ure and  resistance,  83-84 ;  effect  of,  flow- 
•  ing  near  magnetic  needle,  119-120;  induc- 
tion, 149;  measurement  of  electrical,  182- 
198;  rectified,  214;  eddy  or  Foucault,  220; 
laws  the  same  for  direct  and  alternating, 
236 ;  continuous,  236-238  ;  pulsating,  238- 
239;  electric  flow  compared  to  flow  of 
water  from  pumps,  241-242;  product  of, 
and  pressure,  248-249 ;  effective,  249-250 ; 
effect  of  self-induction  caused  by  magnetic 
field  created  by,  255-256;  small,  used  in 
telegraphy,  339;  earth,  414;  in  electro- 
plating, 433-434.  See  Alternating  current. 

Current  electricity,  3. 

Currents,  electric,  exist  in  muscles  and  nerves 
of  animals,  114;  direct,  241;  polyphase, 
268-269. 

Curve  of  magnetization,  129-130. 

Curves,  of  magnetization  of  soft  iron,  130-131 : 


permeability,  of  wrought  iron,  soft  steel 
castings,  and  cast  iron,  132;  calibration, 
of  a  galvanometer,  161-162 ;  of  pulsating 
current,  238,  246,  247;  of  alternating  cur- 
rents, 239,  240,  242,  247,  248,  249 ;  of  lag 
in  alternating  currents,  256,  259,  260;  to 
illustrate  currents  in  two-phase  system, 
268;  in  three-phase  system,  268;  candle- 
power,  279;  load,  of  electric  light  station, 
315-316;  starting  current,  318-319; 
cyanide  solutions,  432,  436. 


Daniell's  battery,  41-42. 

D'Arsonval  galvanometer,  160,  161. 

Davy,  Sir  Humphry,  129,  335 ;   exhibition  of 

electric  arc  by,  273. 
Dead-beat  galvanometer,  160-161. 
Declination  of  magnetic  needle,  65,  79. 
Decomposition  of  electrolytes,  53-54. 
Demagnetization,  68-69. 
De  Meritens,  arc  welding  process  first  used 

by,  449. 

Density,  magnetic,  78. 
Depolarization,  of  electric  battery  cell,  34, 

365;    mechanical,  36-37;    chemical,  37- 

41;  electrochemical,  41-43. 
Depretz,  first  chemical  use  of  electric  arc 

by,  443- 

Deptford  Central  Station,  London,  381-382. 

Dial  pattern  of  bridge,  175. 

Diamagnetic  materials,  67,  133. 

Dielectric  defined,  25,  459. 

Differential  duplex,  344. 

Diffusion  of  two  solutions,  43. 

Dioxide  of  manganese  used  in  chemical  de- 
polarization, 37,  40-41. 

Dip  needles,  64-65. 

Direction,  of  magnet  in  magnetic  field,  77; 
of  field  around  electric  current,  122;  of 
induced  pressure  in  moving  wire,  142-143. 

Discharging  storage  battery,  process  of,  46, 

47-  48. 

Distribution  of  electric  power,  382-388; 
series  and  parallel  systems  of,  390; 
multiple  series  systems  of,  391 ;  three- 
wire  system  of,  391-393 ;  five-wire  system 
of,  393  ;  alternating  current  system  of,  394. 

Divided  circuits,  laws  of,  95,  163. 

Divided  wire  bridge,  176-177. 

Drop,  in  conductors  of  parallel  lighting  sys- 


INDEX 


473 


terns,  288-289;  telephone  switchboard, 
359>  S^o;  in  transmission  wire,  deter- 
mination of,  387-388. 

Drum  armature,  218-219. 

Dry  batteries,  43-44. 

Dufay,  2. 

Dynamo,  discovery  of  principle  of  operation 
of,  139;  direct  current,  212;  single  coil, 
212-213 ;  series-wound,  226-227 '»  shunt- 
wound,  227-228  ;  compound-wound,  227 ; 
multipolar,  231-232 ;  consequent-pole, 
232 ;  features  required  for  good,  234 ; 
alternating  current,  245  (see  Alterna- 
tors) ;  Brush  arc-light,  280-281 ;  Thom- 
son-Houston arc-light,  281 ;  Wood 
arc-light,  282 ;  early  Edison  ("  spindle- 
shank"),  292-293;  "Jumbo,"  294; 
modern,  294-295 ;  cutting  out  of  circuit, 
305-307. 

Dynamos,  batteries  more  expensive  than, 
38-39,  46 ;  transformers  compared  with, 
264;  used  in  electroplating,  433-434. 

E 

Earth,  electric  potential  of,  considered  as 
zero,  17 ;  electric  field  of  the,  18 ;  mag- 
netic condition  of,  63,  79-80 ;  magnetism 
of,  constant  at  any  fixed  position,  156. 

Earth  currents,  414. 

Eddy  currents,  220,  263. 

Edison,  Thomas  A.,  present  form  of  incan- 
descent lamp  due  to,  283 ;  invents  micro- 
phone, 356-357. 

Edison  electrolytic  meter,  166,  182,  203. 

Edison  tubing,  380. 

Edison-Lalande  cell,  39-40. 

Efficiency  of  cell  of  storage  battery,  47. 

Electric  field  of  the  earth,  18. 

Electricity,  origin  of  the  word,  i ;  nature  of, 
1-2;  Franklin,  Dufay,  and  Symmer's 
theories  concerning,  2 ;  properties  of,  3 ; 
positive  and  negative,  3-4 ;  unit  quantity 
of,  called  a  "  coulomb,"  12 ;  can  be  recog- 
nized only  by  its  effects,  16;  friction 
machines  for  generating,  18-19 1  voltaic 
or  galvanic,  32;  amount  of  chemical 
action  in  cell  depends  on  amount  of,  45 ; 
close  relationship  between  magnetism 
and  current,  74 ;  flow  of,  82-101,  236-242. 

Electrochemical  action,  law  of,  45. 

Electrochemical  equivalent,  45. 


Electrochemical  equivalents,  table  of,  56. 
Electrodes,  defined,   32;    for  dry  batteries, 

43-44;  of  electrolytic  cell,  51. 
Electrodynamometer,  187-188. 
Electrolysis,  defined,  51 ;  of  acidulated  water, 

57;  theory  of,  58-68 ;  commercial,  430-431. 
Electrolyte,  the,  defined,  32;  decomposition 

of.  53-54- 

Electrolytic  dissociation,  theory  of,  58. 
Electromagnet  defined,  128. 
Electromagnetic  field,  direction  and  strength 

of  an,  120-122. 

Electromagnetism,  119-135;  defined,  120. 
Electrometallurgy,  430-431. 
Electrometer,  defined,   13;    quadrant,   194- 

195,  204. 
Electromotive  force,  17,  32 ;    unit  of,  30-31 ; 

effective,  249-250;  counter,  322-323. 
Electrophorus,  19-20. 
Electroplating,  57,  431-439. 
Electroscope,  10. 

Electrostatic  induction,  6-8,  11-12. 
Electrotherapeutics,  primary  batteries  used 

in,  46.     See  Roentgen  rays. 
Electrotyping,  437-438. 
Energy,  conservation  of,  107. 
Equivalent  weights,  54. 
Equivalents,  chemical,  54;   electrochemical, 

45.  56. 

Ether,  defined,  2 ;  waves,  456-458. 
Euripides,  mention  of  magnet  by,  63. 
Ewing,  Professor  J.  A.,  130-131. 
Exchanges,  telephone,  358. 
Excitation  of  field  of  alternators,  266-267. 
Exciter,  266-267. 

Exhaustion  of  incandescent  lamps,  283-284. 
Experiments  with  pith  balls,  amber,  sealing- 
wax,  glass  rod,  3. 


Fan,  electric,  320-321. 

Farad  defined,  24. 

Faraday,  Michael,  laws  of,  concerning  de- 
composition of  electrolytes,  53-54 ;  dis- 
covers that  pressure  is  induced  by  mov- 
ing conductor  across  a  magnetic  field, 
138-139  ;  may  be  considered  primary 
inventor  of  dynamo,  212;  quoted  con- 
cerning electromagnetic  inertia,  243-244. 

Farmer,  282. 

Faure  plates,  48. 


474 


INDEX 


Feeders,  in  parallel-running  electric  stations, 
304 ;  in  distributing  system  of  electric 
plant,  382;  arrangement  of,  and  mains, 
for  large  buildings,  406,  407. 

Fibre  for  needle  support,  158. 

Field,  Cyrus  W.,  349-350. 

Field  offeree,  magnetic,  75. 

Field  magnets,  221. 

Filaments,  production  of  carbon,  284-285. 

Fire,  risk  of,  from  ground  returns,  371-372. 
See  Underwriters,  rules  of. 

Five-wire   system   of  electrical  distribution, 

393- 

Fluorescence,  463. 

Fluoroscope,  465. 

Flux,  magnetic,  denned,  78. 

Foot  pounds  denned,  104. 

Force,  coercive,  69;  exerted  between  two 
magnetic  poles,  74;  lines  of,  77;  of  at- 
traction or  repulsion  between  two  bodies 
is  mutual,  77;  magnetomotive,  78.  See 
Electromotive  force. 

Forging,  electric,  444-451. 

Forming,  process  of,  in  construction  of  lead 
cells,  48. 

Foucault  currents,  220,  263. 

Franklin,  Benjamin,  theory  of,  concerning 
electricity,  2;  demonstrates  identity  of 
electrical  discharges  and  lightning,  23; 
electric  chimes  invented  by,  59-60. 

Frequency  of  alternating  current,  253. 

Fur,  attitude  toward  other  substances  when 
electrically  charged,  5. 

Fuse,  electric  light,  407-408 ;  Edison  plug, 
408. 

G 

Gains,  369. 

Galvani,  28  ;  discovers  physiological  effect  of 
electric  current,  113-114. 

Galvanic  battery,  32. 

Galvanizing,  439. 

Galvanometer,  156-163;  sine,  157;  tangent, 
157;  reflecting,  157-158 ;  D'Arsonval,  160- 
161 ;  constant  of  the,  161-162;  ballistic, 
208;  use  of  a  dynamo,  306;  sensitive,  in 
testing  telegraph  and  telephone  lines,  412 ; 
differential,  used  as  ground  detector,  422. 

Gas,  coulomb  of  electricity  roughly  equivalent 
to  cubic  foot  of,  13. 

Gassner,  43. 

Geissler  pump,  284. 


Generators,  dynamo  electric,  see  Dynamos; 
magneto  electric,  see  Magnetos. 

German  silver  for  resistance  boxes,  170. 
See  Alloys. 

Gilbert,  Dr.  William,  i,  2,  63. 

Gilding  insides  of  vessels  by  electricity,  436. 

Glass,  attitude  of,  toward  other  substances, 
when  electrically  charged,  5 ;  conducting 
powers  of,  6;  effect  of  temperature  on 
resistance  of,  88 ;  specific  inductive  capac- 
ity of,  205 ;  used  for  insulators  for  elec- 
tric lines,  374-375. 

Gold,  value  of,  as  electrochemical  equivalent, 
56 ;  rank  of,  as  a  conductor,  83. 

Gold  leaf  used  in  electroscope,  10. 

Gold  plating,  436. 

Gordon,  J.  E.,  quoted,  291. 

Gramme,  212,  216. 

Gramme,  definition  of,  57;  armature,  216- 
217. 

Graphite,  relative  conducting  power  of,  6. 

Gray,  Dr.  Elisha,  349,  352. 

Grids  defined,  48. 

Grotthus,  theory  of  electrolytic  dissociation 
of,  58-60. 

Ground  detector,  421-423. 

Ground  plates  of  telegraph  line,  339. 

Ground  return,  in  telephone  circuit,  362-363  ; 
risk  of  fire  from,  in  electric  light  circuit, 
371-372. 

Ground  of  telegraph  or  telephone  line,  410 ; 
location  of  a,  415-417. 

Grove's  cell,  38-39. 

Gutta-percha,  attitude  of,  when  electrically 
charged,  toward  other  substances,  5; 
specific  inductive  capacity  of,  205 ;  for 
insulating  ocean  cables,  350. 

Guy,  telegraph  pole,  372. 

Gymnotus,  capability  of,  of  delivering  elec- 
tric shock,  114. 


Hall,  discovery  of  electrical  process  of  refin- 
ing aluminum  by,  441. 

Heat,  effect  of,  on  magnet,  68-69 :  effect  of, 
on  resistance  of  materials,  87-88;  pro- 
duced by  current  passing  through  a  wire, 
112;  in  armature  cores  due  to  hysteresis, 
220;  causes  "burning  out"  of  dynamos, 
231 ;  produced  by  alternating  current, 
247-248  ;  light  produced  by  means  of,  by 


INDEX 


475 


current  flowing  through  wire,  282 ;  caused 
in  incandescent  circuits  by  poor  connec- 
tions, 418-419.  See  Thermo-electricity. 

Heaters,  electric,  451. 

Helmholtz,  H.  L.  von,  244. 

Henry,  Joseph,  138-139,  236 ;  discovers  elec- 
tro-magnetic inertia,  243;  electric  tele- 
graph a  growth  from  discoveries  of,  335 ; 
principle  of  electric  bell  first  made  use  of 
by,  367. 

Heroult,  discovery  of  electrical  process  of 
refining  aluminum  by,  441. 

Hertz,  Heinrich,  production  of  electromag- 
netic waves  by,  458-460. 

Hoist,  electric,  322,  323. 

Holtz  induction  machine  described,  20-21. 

Horse-power  defined,  105. 

Horse-power  hour  (H.P.H.),  105,  210. 

Horseshoe  magnets,  68. 

Horseshoe  electromagnets,  128,  129. 

Hot  wire  electrical  measuring  instruments, 
189,  250-252. 

Hoyt  wattmeter,  201-202. 

Hughes,  D.  E.,  356. 

Hydrogen,  value  of,  as  electrochemical  equiv- 
alent, 56 ;  separation  of,  from  acidulated 
water,  57-58  ;  proportion  between  weight 
and  bulk  of,  compared  with  oxygen,  58 ; 
electrochemical  equivalent  of,  166. 

Hydrogen  gas,  created  in  electric  battery 
cell,  34;  amount  of,  liberated  from  elec- 
trolyte, 45. 

Hysteresis,.  130-13 1 ;  heat  in  armature  cores 
-"due  to,  220. 

I 

Illumination,  intensity  of,  varies  inversely  as 
square  of  distance  from  light,  425  ;  meas- 
ure of,  426-428 ;  rule  for  intensity  of,  in 
candle  feet,  427 ;  suitable  for  reading,  427. 

Immersion,  plating  by  simple,  434. 

Impedance,  electrical,  256-257,  448. 

Incandescent  lamps,  measurement  of  power 
used  in,  260;  invention  of,  282-283;  ex~ 
hausting,  283-284;  parallel  and  series 
connections  for,  compared,  286-287; 
effect  of  change  of  pressure  on,  288 ; 
multiple  series  system  for,  391 ;  used  in 
testing  arc  circuits,  420-421 ;  in  testing 
constant  pressure  circuits,  421-423. 

Inclina'ion  of  magnetic  needle,  79. 

India-rubber,  attitude  of,  when  electrically 


charged,  toward  other  substances,  5; 
conducting  powers  of,  6.  See  Rubber. 

Indicator,  ether  wave,  460. 

Induction,  charging  bodies  by,  6-7 ;  electro- 
scope charged  by,  10-11;  magnetizing 
by,  66 ;  electromagnetic,  138-154 ;  mutual, 
149 ;  electromagnetic,  for  intensifying 
effect  of  telephone  transmitter,  357 ;  elec- 
tromagnetic and  electrostatic  may  cause 
cross  talk  between  telephone  wires,  412. 

Induction  coils,  149-151;  in  telephone 
transmitter,  357. 

Induction  currents,  149. 

Inductor  alternators,  266. 

Inertia,  electromagnetic,  243-244;  effect  of, 
on  flow  of  alternating  currents,  253-255. 
See  Self-induction. 

Insulation,  of  telegraph  and  telephone  wires, 
371-372 ;  of  electric  lines,  374-376 ;  of 
light  and  power  cables,  380-382;  cause 
of  fall  in  quality  of,  398 ;  measurements, 
414-415. 

Insulators,  5;  become  conductors  when 
heated  red-hot  or  melted,  88 ;  for  tele- 
phone cables,  205 ;  for  bell  wire,  364 ; 
glass  telephone,  369,  370;  porcelain,  375, 
376-  395-  39° :  fibrous,  376. 

Intensity  of  earth's  magnetic  field,  79. 

Ions  defined,  51. 

Iron,  value  of,  as  electrochemical  equivalent, 
56;  most  strongly  magnetic  material 
known,  67;  coercive  force  of,  69;  rank 
of,  as  a  conductor,  83;  magnetic  perme- 
ability of,  132-133;  residual  magnetism 
in,  232;  losses  in  transformers,  262-263; 
\\  ires  of,  used  for  telegraph  and  telephone 
lines.  370. 

Iron  filings,  experiment  with,  and  magnet, 
70,  76;  experiment  with,  to  show  mag- 
netic field  surrounding  current,  120; 
illustration  of  magnetic  field  within  sole- 
noid, 125. 

Joints,  wire,  373-374;  in  elecric  light  wires 
are  soldered,  418 ;  welding  street  railway, 
447-448. 

Joule,  James  Prescott,  104,  no. 

Joule,  the,  defined,  104 ;  relation  of  the  calorie 
to,  no. 

Joule's  Law,  no. 

Jumbo  dynamo,  294. 


4/6 


INDEX 


Keeper  for  magnet,  69,  70. 

Kelvin,  Lord,  116, 188  ;  studies  of,  in  electro- 
magnetic induction,  244;  ocean  cable 
receiving  apparatus  designed  by,  350. 

Kelvin  balance,  188. 

Key,  telegraph,  337-338. 

Kilo,  the  prefix,  defined,  211. 

Kilowatt  defined,  106. 


Lag,  alternating  current,  244,  256-257. 

Lalande,  39. 

Lamp,  galvanometer,  157-158.  See  Arc 
lamps  and  Incandescent  lamps. 

Lathe,  dentist's,  driven  by  electric  motor, 
321,  322;  machinist's,  330,  331. 

Launches,  electric,  332-333. 

Law,  of  electrochemical  action,  45 ;  Ohm's, 
83-84,  100,  loi,  169,  171-172,  179,  244- 
245,  256-257,  337 ;  for  fall  of  potential  in 
circuit,  101 ;  Lenz's,  151-152. 

Laws  of  Faraday,  53-54 ;  application  of,  54- 
56. 

Lead,  plates  of,  used  in  storage  cells,  47; 
cells,  construction  of,  48  ;  sulphate  of,  for 
pasting  plates,  48 ;  value  of,  as  electro- 
chemical equivalent,  56;  rank  of,  as  a 
conductor,  83 ;  coverings  of,  for  under- 
ground cables,  377. 

Leak  caused  by  poor  insulation,  374. 

Leakage,  magnetic,  221. 

Leclanche  cells,  40-41. 

Lenard,  Philip,  464. 

Lenz,  116. 

Lenz's  Law,  151-152. 

Level,  difference  of,  16;  of  earth  considered 
as  zero,  17. 

Leyden  jar,  26. 

Lighting,  arc,  273-281;  incandescent,  281- 
289;  wires  for  electric,  made  of  electro- 
lytic copper,  439.  See  Arc  lamps  and  In- 
candescent lamps. 

Lightning,  identity  of  electrical  discharges 
and,  23-24. 

Lights,  distribution  of,  425-426. 

Lines  of  magnetic  force,  77. 

Liquids,  effect  of  temperature  on  resistance 
of,  88. 

Litharge  used  in  process  of  pasting  plates, 
48. 


Load   curve   of  electric  light  station,  315- 

316. 

Loads,  station,  314-316. 
Local  action  in  battery  cells,  44-45. 
Locomotive,  electric,  313. 
Lodestones,  63. 
Loop  method  for  locating  fault  in  electric 

line,  416-417. 
Loss,  hysteresis,  130-131,  220;    of  pressure 

in  parallel-lighting  systems,  288-289;   °f 

power  in  belting  and  shafting,  329. 
Losses,  transformer  iron,  262-263. 

M 

Machines,  for  generating  electricity,  18-21 ; 
arc-lighting,  280-281 ;  cutting  out  of  cir- 
cuit, 305-307.  See  Dynamos,  Motors. 

Magnet,  derivation  of  the  word,  63 ;  satura- 
tion of,  69;  ageing  of,  69-70;  laminated, 
71;  continuous  motion  produced  in,  by 
electric  current,  138  ;  dynamo  field,  221 ; 
differential,  277;  series,  277;  of  Bell  tele- 
phone, 353-354. 

Magnetic  vane  instrument,  190-191 ;  used  in 
alternating  current  measurements,  252. 

Magnetism,  nature  and  properties  of,  63-80; 
temporary  and  permanent,  64;  induced, 
66 ;  theories  about  phenomena  of,  72-74 ; 
Ampere's  theory  of,  73,  125 ;  close  rela- 
tionship between  current  electricity  and, 
74;  terrestrial,  79-80;  residual,  128,  232- 
233  ;  relation  between  ampere  turns  and, 

ISO- 
Magnetite,  63. 

Magnetization,  curve  of,  129-130. 
Magnetomotive  force,  78. 
Magnetos,  139,  357  ;  telephone,  213  ;  for  arc- 
line  testing,  419. 
Magnets,  artificial,  64;  bar  and  horseshoe, 

68;    controlling,   of  galvanometer,    159; 

permanent,  for  amperemeters,  185-187. 
Mains,   in    distributing    system    of   electric 

plant,  382;  arrangement  of,  and  feeders, 

for  large  buildings,  406,  407. 
Man,  282,  283. 
Manganese,   dioxide  of,  used   in   chemical 

depolarization,   37,    40-41 ;    a    magnetic 

material,  67. 
Manholes,  underground  conduit,  378. 
Manufactories,  electric  motors  in,  329-332. 
Maps,  magnetic,  80. 


INDEX 


477 


Marconi,  William,  463. 

Maxim,  282. 

Maxwell,  Clerk,  accepts  Weber's  theory  con- 
cerning magnetism,  73;  on  electromag- 
netic inertia,  244 ;  demonstrates  possibil- 
ity of  electromagnetic  waves,  458. 

Measurement,  of  currents  and  pressures,  182- 
198 ;  of  electric  pressure  by  comparison, 
196-197 ;  of  electric  power,  200-204 ;  of 
pressure  of  static  charge,  204 ;  of  capacity 
by  ballistic  galvanometer,  208-210;  of 
alternating  electric  pressure,  249-250 ;  of 
power  in  alternating  circuit,  259-261 ; 
of  power  used  in  incandescent  lamp,  260 ; 
daily  or  weekly,  of  trunk  telegraph  or 
telephone  lines,  412;  of  candle  power, 
424-425;  of  illumination,  425-428. 

Medicine,  electricity  used  in,  114-115. 

Meg,  the  prefix,  defined,  211. 

Megohm  defined,  178. 

Mercury,  used  for  amalgamating  zinc,  44; 
rank  of,  as  a  conductor,  83;  used  for 
producing  vacuum  in  incandescent  lamp 
bulbs,  284. 

Metallic  circuit,  measuring  conductivity  of, 
412-413. 

Metals,  attitude  of,  when  electrically  charged, 
toward  other  substances,  5 ;  relative  con- 
ducting power  of,  6;  become  charged 
when  dipped  in  certain  liquids,  28 ;  used 
in  Volta's  pile,  30 ;  conducting  powers  of, 
82-83;  sa-lts  °f.  431:  electric  forging  of, 
444-451 ;  list  of,  which  have  been  welded 
by  Thomson  process,  447. 

Meter,  Edison  electrolytic,  166,  182,  203. 

Meter  bridge,  176-177. 

Mica,  conducting  powers  of,  6;  specific  in- 
ductive capacity  of,  205  ;  as  insulator  of 
segments  of  commutator,  231. 

Micro,  the  prefix,  defined,  211. 

Microamperes  defined,  183. 

Microfarads,  25,  207. 

Microphone,  355-357. 

Microvolts  defined,  116. 

Mil,  definition  of,  90;  the  circular,  89-90. 

Milli,  the  prefix,  defined,  211. 

Milliamperemeters,  defined,  183 ;  for  deter- 
mining line  insulation,  414-415. 

Milliamperes  defined,  183. 

Minium  used  in  process  of  pasting  plates, 
48. 


Mirror,  galvanometer,  157-158. 

Moissan  quoted  concerning  production  of 
calcium  carbide,  443. 

Molecules,  magnetic,  72-73. 

Momentum,  electromagnetic,  243-244. 

Morse,  S.  F.  B.,  335-336. 

Morse  alphabet,  337,  340. 

Motors,  electric,  224-225;  street  railway, 
230-231,  309-310;  iron-clad,  230;  syn- 
chronous, 267-268;  induction,  269-270; 
uses  of  stationary,  319-322;  counter  elec- 
tric pressure  of,  322-323  ;  starting  box  or 
rheostat  of,  324-327;  starting  and  stop- 
ping, 327-328;  reversing,  328-329;  in 
manufactories,  329-332.  See  also  Dyna- 
mos. 

Mouldings  for  carrying  electric  light  wires, 
402. 

Multiple  arc,  connection  in,  97. 

Multiple  series  system  of  electric  lighting, 


Nature  of  electricity,  1-2. 

Needle,  magnetic,  64-65 ;  experiment  with 
floating  magnetized,  and  magnet,  75-76 ; 
effect  of  current  flowing  near  magnetic, 
119-120;  supports  of,  in  galvanometer, 
158;  astatic,  159;  galvanometer,  159-160; 
throw  of  galvanometer,  209. 

Needle  telegraph,  337. 

Negative  charge  defined,  18. 

Nernst  lamps,  88. 

Niagara  Falls  Power  Company's  plant,  fre- 
quency used  at,  253;  description  of,  298- 

3°4- 

Nickel,  value  of,  as  electrochemical  equiva- 
lent, 56 ;  a  common  magnetic  material, 
67 ;  rank  of,  as  a  conductor,  83. 

Nickel  plating,  436-437. 

Nickel  salts,  436. 

Nitrate  of  copper,  51,  52. 

Nitrate  of  silver,  431. 

Nitric  acid,  used  in  chemical  depolarization, 
37;  should  not  come  in  contact  with  zinc, 
38  ;  action  of,  as  depolarizer  more  power- 
ful than  bichromate  of  potash,  39;  used 
in  silver  plating,  431. 

Nitrogen,  value  of,  as  electrochemical  equiva- 
lent, 56. 

Non-conductors  defined,  5. 


478 


INDEX 


0 

Oersted,  Hans  Christian,  115,  116,  236,  335. 

Ohm,  Dr.  Georg  Simon,  84. 

Ohm,  the  international,  86. 

Ohm's  Law,  83-84,  loo,  101,  169,  171-172, 
179;  modified  for  general  application, 
244-245 ;  applied  to  flow  of  alternating 
currents,  256-257 ;  of  service  in  develop- 
ment of  telegraphy,  337. 

Oils,  conducting  powers  of,  6. 

Open  circuit  cells,  34-35;  Leclanche  cells 
are,  41. 

Open-work  wiring  defined,  399. 

Oscillator,  458. 

Oxygen,  value  of,  as  electrochemical  equiva- 
lent, 56 ;  separation  from  acidulated  water, 
57-58 ;  proportion  between  weight  and 
bulk  of,  compared  with  hydrogen,  58. 


Pacinotti,  212. 

Pail  welding,  450-451. 

Paper,  crinkled,  for  insulating  telephone 
cables,  205,  377. 

Paraffine,  conducting  powers  of,  6 ;  specific 
inductive  capacity  of,  205. 

Paraffine  oil  used  in  copper  oxide  cell,  39. 

Parallel,  plates  connected  in,  49;  condensers 
connected  in,  206-207;  alternators  con- 
nected in,  267;  incandescent  lamps 
usually  connected  in,  286;  connection 
in,  for  electrical  distribution,  390. 

Parallel  circuit,  93-95 ;  combined  with  series 
circuit,  97-98. 

Paramagnetic  materials,  67,  132-133. 

Pasting,  process  of,  48. 

Pearl  Street  Central  Station,  New  York,  380. 

Peltier,  115-116. 

Peltier  effect,  116. 

Penstocks,  299. 

Period  of  alternating  current,  253. 

Permeability,  magnetic,  131-133. 

Peroxide  of  lead  in  storage  cell,  47. 

Petroleum,  specific  inductive  capacity  of,  205. 

Petticoat  for  insulators,  375,  395,  396. 

Phase  of  electric  flow,  240. 

Photometer,  Bunsen,  424;  Weber,  428. 

Physiology,  electricity  in,  113-117. 

Pins,  electric  line,  369,  370. 

Pith  balls,  3 ;  as  electroscopes,  10. 

Plante  plates,  48. 


Plants,  conducting  powers  of,  6. 

Plants,  electric,  development  of,  291-294; 
in  small  cities,  297-298;  the  Niagara, 
298-304;  management  of,  313-316. 

Plates,  of  condenser,  25  ;  Faure,  48  ;  Plante 
48. 

Plating,  see  Electroplating. 

Platinum,  a  magnetic  material,  67 ;  rank  of, 
as  a  conductor,  83 ;  used  in  incandes- 
cent lamps,  285. 

Plato,  mention  of  magnet  by,  63. 

Plumbago  used  in  electrotyping,  438. 

Plunge  battery,  37.  . 

Poisson,  Simeon  Denis,  theories  of,  con- 
cerning magnetism,  72. 

Polarization,  of  electric  battery  cell,  34;  in 
molecules  of  magnetic  material,  72. 

Pole  changer,  346. 

Poles,  of  electric  battery  cell,  32;  of  mag- 
netic needle,  64-65 ;  every  magnetic 
body  contains  two,  of  opposite  signs,  67; 
unit  magnet,  74. 

Poles,  electric  line,  369,  370;  erection  of, 
372.  _ 

Porcelain,  conducting  powers  of,  6  ;  used  for 
insulators  in  electric  lines,  375,  376,  395, 
396. 

Positive  charge  defined,  18. 

Post-office  pattern  bridge,  174 ;  applied  to 
loop  test  for  locating  fault  in  electric  line, 
417. 

Potash,  bichromate  of,  used  in  chemical  de- 
polarization, 37 ;  nitric  acid  more  power- 
ful than,  39. 

Potential,  difference  of,  16;  of  the  earth  con- 
sidered as  zero,  17;  fall  of,  in  circuit, 

IOO-IOI. 

Potentials,  relative,  17-18. 

Potentiometer,  197. 

Power,  defined,  105 ;  loss  of,  in  belting  and 
shafting,  329;  electrical  distribution  of, 
387-388. 

Pressure,  electrical,  17,  22;  of  cell  indepen- 
dent of  its  size,  33-34;  magnetic,  78;  fall 
of,  along  a  circuit,  100-101 ;  relation  of, 
and  charge  and  capacity  in  a  condenser, 
205-206;  product  of,  and  current,  248- 
249;  effective,  249-250;  effect  of  change 
of,  on  incandescent  lamp,  288 ;  electric 
railway,  310-311 ;  counter  electric,  322- 
323 ;  in  electroplating,  433. 


INDEX 


479 


Pressure  indicators,  193. 

Primary  batteries,  36-45. 

Primary  coils,  147. 

Prism  Leclanche  battery,  40,  41. 

Protoplasm,  effect  of  electric  current  on,  114. 

Pump,  electrical  machine  compared  to,  21 ; 

analogy  applied  to  internal  resistance,  84 ; 

analogy  as  applied  to  series  circuit,  92 ; 

analogy  drawn  between  electric  current 

flow  and  flow  of  water  from,  241-242 ; 

Geissler  vacuum,  284;  Sprengler  vacuum, 

284 ;  electric,  322. 

q 

Quadrant  electrometer,  194-195,  204. 
Quicking,  in  silver  plating,  435. 


Radiation  of  heat  from  wire,  112. 

Radiographs,  465,  466. 

Railways,  electric,  early  history  of,  307-308 ; 
principle  of,  308-309;  for  heavy  service, 
312-313. 

Ratio  of  transformation,  263-264. 

Ratio  arms  of  Wheatstone  bridge,  173. 

Receiver,  Bell  telephone,  353. 

Recorder,  siphon,  350. 

Refining  of  copper,  439-440 ;  of  other  metals, 
440-441. 

Register,  recording  telegraph,  339-340. 

Relay,  telegraph,  336,  342,  343;  differential, 
344;  polarized,  345-346;  neutral,  346. 

Reluctance,  magnetic,  133-134;  of  magnetic 
circuit  of  a  dynamo,  221-222. 

Repulsion,  force  of,  between  two  charged 
bodies,  12;  magnetic,  65-66;  force  of,  be- 
tween two  bodies  is  mutual,  77 ;  mutual, 
of  electric  circuits,  154. 

Residual  magnetism,  128,  232-233. 

Resinous  substances,  attitude  toward  other 
substances  when  electrically  charged,  5. 

Resistance,  electrical,  22;  unit  of,  84;  inter- 
nal, 84;  standard  of,  85-86;  of  similar 
wires  varies  as  squares  of  their  diameters, 
86 ;  definition  of  the  specific,  of  a  material, 
90 ;  of  a  wire  depends  directly  on  length 
and  inversely  on  cross  section,  90;  of 
conductors  varies  greatly,  91 ;  of  divided 
circuits,  95 ;  power  used  in  overcoming, 
107-108 ;  of  galvanometer  shunts,  162- 
163 ;  measurement  of,  169-180 ;  volt  and 


current  method  of  measuring,  179-180; 

apparent,  256. 

Resistance  boxes,  169-170.    See  Rheostat. 
Resonator,  Hertzian,  459-460. 
Retardation   of  electric  current,   244.      See 

Inertia,  electromagnetic. 
Rheostat,  169;    field,  229;    necessity  for,  in 

field  of  alternator  and  exciter,  269 ;  start- 
ing, for  motors,  324-327. 
Richmond,  Va.,  early  electric  railway  at,  307- 

308. 
Right-hand  rule  for    direction   of   induced 

current,  143. 
Ring  armature,  216-217. 
Rings,  welding  of,  448-449. 
Rodding  cable  duct,  379. 
Roentgen,  William  Konrad,  464-466. 
Roentgen  rays,  464-468. 
Rotary  converters,  270-271. 
Rowland,  Henry  Augustus,  no.. 
Rubber,  hard,  a  non-conductor  of  electricity, 

5 ;    specific    inductive    capacity   of,   205 ; 

used  for  insulators  in  electric  lines,  375. 

See  India-rubber. 
Ruhmkorff  coils,  150. 


Safe  carrying  capacity  of  wires,  table  of,  399. 

Safety  fuses,  398. 

Sal  ammoniac,  solution  of,  used  in  electric 
battery  cells,  37;  in  chloride  of  silver 
battery,  40. 

Salts,  of  copper,  51-52;  of  silver,  431; 
nickel,  436. 

Salty  solutions,  relative  conducting  power 
of,  6. 

Saturation  of  magnet,  69-70. 

Sawyer,  282,  283. 

Scales,  amperemeter,  189-190. 

Schweiger,  335. 

Screens,  electric,  15. 

Screw  and  nut  illustration  of  direction  of 
field  around  a  current,  122. 

Screw  rule  for  direction  of  field  about  a  cur- 
rent, 122. 

Secondary  batteries,  46-47. 

Secondary  coils,  147. 

Sectors  of  Holtz  machine,  20. 

Seebeck,  Thomas  Johann,  115. 

Self-induction,  effect  of,  on  flow  of  alternat- 
ing currents,  253-255 ;  caused  by  mag- 


480 


INDEX 


netic  field  created  by  current,  255-256. 
See  Inertia,  electromagnetic. 

Series,  connection  in,  33;  circuits  in,  92; 
circuit,  combined  with  parallel  circuit, 
97-98  ;  condensers  connected  in,  206-207  '•> 
arc  lamps  usually  connected  in,  278 ;  con- 
nection in,  for  electrical  distribution,  390- 
39i. 

Series-parallel  control,  317-319. 

Sewing  machine,  electric,  321,  322. 

Shell,  magnetic,  68. 

Shellac,  conducting  powers  of,  6;  specific 
inductive  capacity  of,  205. 

Shock,  electric,  114. 

Short-circuiting,  170-171. 

Shunt  box,  163. 

Shunt  boxes  for  galvanometers,  use  of,  178- 
179- 

Shunts,  defined,  100;  galvanometer,  162-163. 

Siemens,  Si/  William,  212;  chemical  use  of 
electric  arc  by,  443. 

Siemens  armature,  218-219. 

Siemens  electrodynamometer,  187-188. 

Silk,  attitude  of,  when  electrically  charged, 
toward  other  substances,  5 ;  conducting 
powers  of,  6. 

Silver,  value  of,  as  electrochemical  equivalent, 
56 ;  ranks  with  copper  as  best  conductor 
known,  82-83  '<  nitrate  of,  for  voltameter, 
165-166;  electrochemical  equivalent  of, 
166;  salts  of,  431. 

Silver  chloride  battery,  40. 

Silver  plating,  431-435. 

Sine  galvanometer,  157. 

Size  of  wire  in  light  and  power  systems, 
determination  of,  383-385. 

Sleeve  wire  joint,  373-374. 

Slide  wire  bridge,  176-177. 

Sludge,  440. 

Smee's  cell,  36. 

Smelting,  electric,  442-443. 

Socket  of  incandescent  lamp,  285-286. 

Soda,  bichromate  of,  used  in  chemical 
depolarization,  37. 

Solenoid,  defined,  124-125 ;  magnetizing 
effect  of,  on  magnetic  materials,  127-128. 

Sounder,  telegraph,  337,  341-342. 

Spark  coils,  153. 

Sparking  of  dynamo,  233-234. 

Specific  inductive  capacity,  204-205. 

Spindle-shank  dynamo,  292-293. 


Sprengel  vacuum  pump,  284. 

Spring  jack,  telephone,  359-360. 

Standard  candle,  424. 

Standard  resistance,  178. 

Starting  box,  motor,  324-327. 

Static  electricity,  3 ;  experiments  with,  cannot 
be  made  with  damp  materials,  5 ;  tends 
to  stay  on  surface  of  conductor,  13-14. 

Station,  development  of  central,  291-293; 
Pearl  Street,  New  York,  293-294;  the 
vertical,  295-296;  management  of,  313- 
316  ;  Deptford  Central,  London,  38 1-382. 

Station  loads,  314-316. 

Steel,  coercive  force  of,  69 ;  as  magnet,  69 ; 
welding  of,  by  electrical  process,  446-449. 

Storage  cell,  lead  plate,  48. 

Storms,  magnetic,  79. 

Sturgeon,  William,  129,  335. 

Sulphate  of  copper,  51,  52. 

Sulphide   of  copper,   51. 

Sulphur,  attitude  of,  when  electrically 
charged,  toward  other  substances,  5 ; 
specific  inductive  capacity  of,  205. 

Sulphuric  acid,1  used  in  bichromate  plunge 
battery,  38  ;  in  Daniell's  cell,  41 ;  in 
storage  cells,  47;  defined,  52;  effect  of, 
when  added  to  water,  57. 

Swimming  rule  for  direction  of  field  about  a 
current,  122. 

Switchboard,  arc -lighting,  280-281;  in  cen- 
tral electric  stations,  304-305 ;  telephone, 
359-362. 

Symmer,  2. 

Synchronism  defined,  267. 

Synchronizer,  307. 


Tangent  galvanometer,  157. 

Taps,  insulation  resistance  of  individual, 
should  not  fall  below  100,000  ohms, 
398. 

Telautograph,  Gray's,  349. 

Telegraph,  invention  and  development  of, 
335-337;  the  needle,  337;  submarine, 
349-351- 

Telegraphy,  importance  of  capacity  effects 
in,  26;  gravity  battery  used  in,  41,  46; 
multiple,  342-343;  duplex,  343-345 ;  di- 
plex,  343,  345-347  I  quadruplex,  347-348  ; 
multiplex,  349;  automatic,  349;  auto- 
graphic, 349;  submarine,  349-351;  and 


INDEX 


481 


telephony,  simultaneous,  363-364 ;  troubles 
in,  410-411 ;  wireless,  461-463. 

Telephone,  effect  of  capacity  of  wire  on  use- 
fulness of,  25-26 ;  open  circuit  cells  used 
for,  35;  batteries  useful  for,  46;  history 
and  development  of,  352-353  ;  mechanism 
of,  353-358;  exchanges,  358;  switch- 
boards, 359-362;  same  wire  used  for 
telegraph  and,  363-364. 

Telephone  cables,  insulation  of,  205. 

Telephony,  and  telegraphy,  simultaneous, 
363-364;  troubles  in,  410-412. 

Temperature,  effect  of,  on  resistance  of 
materials,  87-88  ;  of  wire  carrying  current, 
112;  error  in  Wheatstone  bridge,  175. 
See  Thermo-electricity. 

Tests,  of  telegraph  lines,  411-412;  of  tele- 
phone lines,  412. 

Thales,  discovery  of  electrical  property  attrib- 
uted to,  i ;  magnet  mentioned  by,  63. 

Thermo-battery,  116-117. 

Thermo-electricity,  115-117. 

Thermopile,  the,  116-117. 

Thomson,  Elihu,  electric  welding  developed 
by,  445-446. 

Thomson,  Sir  William,  see  Kelvin,  Lord. 

Thomson  alternating  current  amperemeter, 
190-191. 

Thomson  recording  wattmeter,  202-203. 

Three-wire  system  of  electrical  distribution, 

391-393- 

Throw  of  galvanometer  needle,  209. 

Thumb  and  hand  rule  for  direction  of  field 
about  a  current,  123. 

Tin,  value  of,  as  electrochemical  equivalent, 
56;  rank  of,  as  a  conductor,  83. 

Torque  of  series  motor,  317-318. 

Torque  starting,  317-318. 

Track  bond,  312. 

Transformation,  ratio  of,  263-264. 

Transformers,  alternating  current,  150-151, 
261-264;  compared  with  dynamos,  264; 
for  electric  railways,  311 ;  "  step-up,"  396 ; 
of  electric  welders,  445-446. 

Transmission  of  power,  high-pressure  long- 
distance, 394-396. 

Transmitter,  telegraph,  344,  346,  347;  Bell 
telephone,  353  ;  the  Blake  telephone,  354- 
355;  microphone,  354-355;  long-distance, 
363 ;  for  wireless  telegraphy,  459. 

Trolley,  309. 


Trolley  wire,  309. 

Truck,  electric  railway,  310. 

Tube,  Crookes,  463 ;  X-ray,  464. 

Tubing,   Edison,   for   underground    cables, 

380-381. 
Tunnels  of  Niagara  Falls  Power  Company, 

299-300. 
Turpentine,   specific   inductive   capacity  of, 

u 

Underwriters,  rules  of,  concerning  electrical 
wiring,  396-397,  400-401. 

Unit,  magnet  pole,  74 ;  magnetic  field,  77-78  ; 
of  electric  current,  see  Ampere ;  of  elec- 
trical pressure,  see  Volt;  of  electrical 
resistance,  see  Ohm  ;  of  work,  see  Horse 
power. 

Units,  definitions  of  electrical,  adopted  at 
Chicago  congress,  86-87 ;  electrical,  table 
of,  210. 

V 

Vacuum,  magnetic  force  acts  through  a,  67. 

Vacuum  pumps,  284. 

Variation  of  compass  and  dip  needles,  65, 
79-80. 

Vats,  for  silver  plating,  432-433 ;  for  recover- 
ing aluminum  from  alumina,  442. 

Vitreous  electricity,  3. 

Volt,  defined,  22,  28,  30-31 ;  and  current 
method  of  measuring  resistance,  179- 
180;  the  international,  87. 

Volta,  22,  28,  29,  30,  31,  335. 

Volta's  pile,  30;  dry  batteries  represent  a 
return  toward,  44. 

Voltaic  battery,  32. 

Voltaic  cell,  29-30. 

Voltameter,  defined,  51,  164;  water,  57-58, 
164-165;  metal,  165;  silver,  165-166,  431; 
copper  and  zinc,  166;  for  current  meas- 
urement, 182. 

Voltmeter,  180,  191;  the  Weston,  191-192; 
the  Cardew,  193;  electrostatic,  194-195, 
204;  Weston  alternating  current,  250- 
252;  electrostatic,  used  in  alternating 
current  measurements,  252;  Bristol  re- 
cording, 314;  used  in  testing  arc  cir- 
cuits, 420;  in  testing  constant  pressure 
circuits,  421-423. 

Volts  drop  in  transmission  wire,  387. 

Von  Guericke,  18. 

Vulcanite,  conducting  powers  of,  6. 


21 


482 


INDEX 


W 

Watch,  magnetization  of,  by  dynamo,  221. 

Water,  as  a  conductor  5,  6 ;  coulomb  of 
electricity  rough  'y  equivalent  to  gallon 
of,  13  ;  electrolysis  of  acidulated,  57-58  ; 
continuous  currents  compared  to  flow 
of,  236-238 ;  alternating  currents  com- 
pared to  flow  of,  in  tideway,  239-240 ;  elec- 
tric current  flow  compared  to  flow  of,  from 
pumps,  241-242 ;  as  illustration  of  com- 
parative advantages  of  parallel  and  series 
connections  for  lamps  and  motors,  286- 
287 ;  waves  or  vibrations  of,  455-456. 

Water-pipes,  analogy  between  series  circuit 
and,  92;  analogy  between  branched  cir- 
cuit and,  94-95 ;  analogy  between  com- 
pound circuit  and,  98. 

Water  voltameter,  57-58,  164-165. 

Watt,  James,  106. 

Watt,  the,  105-106. 

hours  denned,  203. 

Wattmeter,  200;  the  Hoyt,  200-201;  re- 
cording, 202-204;  integrating,  203,  314; 
alternating  current,  252-253. 

Wave  length  defined,  455. 

Waves,  electromagnetic,  455-460. 

Weber,  theory  of,  concerning  magnetism,  73- 

74- 

Welding,  electric,  444-451. 

Western  Union  wire  joint,  373. 

Weston  amperemeter,  185-187,  191. 

Weston  alternating  current  voltmeter,  250- 
252. 

Wheatstone  bridge,  171-177  ;  in  testing  tele- 
graph and  telephone  lines,  412-414. 

Wheel  pit  of  Niagara  Falls  plant,  300-301. 

Wilson,  Thomas  L.,  one  of  original  discov- 
erers of  calcium  carbide,  443. 


Wire,  sizes  of,  used  on  telegraph  lines,  339 ; 
sizes  of,  for  electric  light  and  power  lines, 
-37°,  383-385;  weather-proof,  371;  data 
concerning  properties  of  copper,  385-387 ; 
neutral,  in  three-wire  system  of  electrical 
distribution,  391 ;  weights  of,  in  the  sev- 
eral systems  of  electrical  distribution,  393- 
394 ;  sizes  of,  for  inside  wiring,  406-407  ; 
locating  grounds  on  electric  light,  423- 
424. 

Wires,  distributing,  288-289  I  f°r  electric  rail- 
ways, 311 ;  stringing  electric  line,  372-373  ; 
insulation  of  electric  line,  374-376 ;  under- 
ground, 376;  safe  carrying  capacity  of, 
399;  for  electric  lighting  and  electric  ma- 
chines made  of  electrolytic  copper,  439. 

Wiring,  underwriters'  rules  concerning,  396- 
397,  400-401 ;  necessity  of  care  in,  401 ; 
open  work,  399;  cleat  and  moulding 
work,  401-402  ;  concealed  work,  402-403  ; 
central  cabinet  plan,  403-405;  crib  sys- 
tem of,  406. 

Wood,  a  non-conductor  of  electricity,  5  ;  at- 
titude of,  when  electrically  charged,  to- 
ward other  substances,  5. 


Zinc,  used  in  Volta's  pile,  30;  plates  of,  used 
in  open  circuit  cell,  37  ;  nitric  acid  should 
not  come  in  contact  with,  38 ;  chemical 
action  on,  by  sulphuric  acid,  44;  value 
of,  as  a  fuel  in  primary  batteries,  45-46 ; 
in  storage  batteries,  46 ;  value  of,  as  elec- 
trochemical equivalent,  56  ;  rank  of,  as  a 
conductor,  83;  electrochemical  equiva- 
lent of,  166 ;  in  voltameters,  166. 

Zinc-carbon  cells  for  batteries,  46. 

Zinc  plating,  439. 


482 


Watch, 

Water, 
elect 
of,  i 
cont 
of,  : 
pare 
trie  i 
pun- 
pare 
con 
287 

Water- 
and 
cuil 
pot 

Water 

Watt, 

Watt, 
XWatt  I 

Wattr 
coi 
alt 

Wave 

Wave 

Webe 

74 

Weld 
West 
West 
Wesl 

Whe 

g 

Whe 
Wils 

e 


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